Electrochemically Assisted Self-Assembly Technique for the

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Electrochemically Assisted Self-Assembly Technique for the Fabrication of Mesoporous Metal-Organic Framework Thin Films; Composing of 3D-Hexagonally Packed Crystals with 2D-Honeycomb-Like Mesopores Saber Alizadeh, and Davood Nematollahi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12564 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 15, 2017

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Journal of the American Chemical Society

Electrochemically Assisted Self-Assembly Technique for the Fabrication of Mesoporous Metal-Organic Framework Thin Films; Composing of 3D-Hexagonally Packed Crystals with 2D-Honeycomb-Like Mesopores Saber Alizadeh and Davood Nematollahi* Faculty of Chemistry, Bu-Ali-Sina University, Hamedan 65174-38683, Iran KEYWORDS: MOF, mesoporous, thin film, electrochemical synthesis, deposition, EASA, metal-organic framework, template, surfactant, Zn3 (BTC)2 ABSTRACT: In this work, we introduce a novel and general strategy for an environmental friendly fabrication of mMOF thin films via electrochemically assisted self-assembly (EASA) technique. Implementation of this procedure as a one-step, additive-free and versatile protocol lead to in-situ simultaneous synthesis and deposition of mesoporous architectures of MOFs at room temperature under green conditions, without need to any base, pretreatment or chemical modification of underlying surface. Our procedure, provides a controllable method for the synthesis of mMOF thin films (modified electrodes) consisted of hollow 3D-hexagonally packed crystals with 2D-honeycomb-like mesopores in the wall of cavity, which grow perpendicularly on to any of conducting surface. The resulting modified electrode showed enhanced electron transfer properties and better mass transfer performance along with the appropriate signal, suitable for electrochemical sensing applications. This work can be a breakthrough and new perspective for the modification and functionalization of the surface with any type of mMOFs by the electrochemical driven co-operative (soft templating) mechanism.

INTRODUCTION Porous materials are spotlight of a comprehensive research area, particularly in material science and chemical industry, due to eye-catching nature and surprising traits.1-4 Zeolites, MCM-n, and SBA-n materials are wellknown materials,5-7 but an upsurge began in the last two decades thanks to introducing of metal-organic framework materials (MOFs) by the pioneering attempts of Yaghi et al.8-13 These materials, consisting of integrated organic-inorganic supramolecular agents including hierarchically crystalline networks of electron donor linkers and electron acceptor metal cations.1,10,14-17 MOFs can be bridged the gap between of zeolites and mesoporous silica materials due to tunable porosity, large surface area and structural diversity.18 A literature survey shows that significant quota of the published papers have been focused on the microporous MOFs (pore size < 2 nm). Limited diffusion and slow mass transfer, as challenging problem in electrochemistry along with low surface area and low loading of guest molecules, are deficient of microporous systems.18 In this context, numerous efforts have been directed toward resolving these problems.19-23 These efforts include the use of extendable organic ligands,21 a combination of mixed ligands23 and bulky secondary building blocks.19 Also, attractive and reliable soft templating or surfactant-templated approach has been adopted as an alternative way to addressing the grave problems like framework instability and collapse, interpenetrating of ligands and pore blocking.24-30 On the oth-

er hand, the most studies concerned with the bulk MOFs, while, utilization of inherent and prominent quality of MOFs as thin films open a new area of important technological applications. In this way, most prominent implementation of MOFs as membranes for separation or sensing devices, require thin films of MOFs.31,32 As a consequence, this fact encouraged researchers to discover various supporting techniques of MOFs, which will display superior performance compared to the bulk crystal materials.32-35 This elegant challenge has significant difficulties because most bulk MOF crystals are brittle and nonmanageable for modification of surfaces with ordinary methods. To overcome this drawback, several methods such as solvothermal,36 microwave induces,37 selfassembled monolayers,38 liquid phase epitaxy,39 evaporation induces40 and gel layer synthesis41 have been developed for the fabrication of MOF thin films. The main drawbacks of these methods are the prolonged time, multi-step, high pressure or vacuum and high temperature conditions for synthesis, and failure of selective production and uniform morphology.31-33,35 In order to access a mild, one-step, selective and controllable method, significant efforts have been dedicated to the development of the electrochemical techniques.13,42 Among of direct/insitu and indirect/ex-situ electrochemical approaches,43 the first method is preferred because the ex-situ deposited films are not intergrown and integrated and their attachment on the surface is often rstricted.44 Anodic and cathodic depositions are the pioneering

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achievements of direct electrosynthesis that recently have been adopted for the fabrication of MOF thin films. In anodic method, anode acts as the source of metal ions and deposition of the film performs near of metal anode.13,42,44-46 In cathodic method, film deposition occurs as a result of the increase in pH near the electrode surface, where electrochemical reduction of probase causes to deprotonation of ligand.47-51 Even though eye-catching and enjoyable advances have been achieved, to the best of our knowledge, all the electrodeposited MOF thin films are restricted to the microporous structures, and there is no reports on the electrochemical synthesis and deposition of mesoporous MOF thin films (mMOFTF) to date. In this way, we envisioned that supporting mMOFTFs on the electrode surface as a modified electrode (mMOFME), provides faster electron transfer kinetics that would be appropriate for sensing applications. It may also facilitate the diffusion of guest molecules onto the active site of electrode thanks to nanoporous structure and enlarged surface area. If this elegant and exciting idea will be accomplished, it will be able to as a breakthrough in MOF employments. So, to shed light on this issue, we propose here a novel and environmental friendly strategy for the fabrication of mesoporous metal-organic framework thin films via electrochemically assisted self-assembly (EASA) technique. Since the introducing of EASA by Walcarious and co-workers,52,53 it has been successfully employed for modification of electrodes in several electrochemical studies.54-56 Inspired by these reports and our experience,57,58 for the first example, EASA is employed for simultaneous synthesis and deposition of MOF thin films with mesoporous architecture. This procedure is conceptually and strategically distinct from the conventional reductive electrosynthesis of MOFs, because of restricted diffusion and slow mass transfer owning to the inherent microporous and low surface area nature, and without paying attention to orientation and morphology. In addition, eco-friendly and energy consumption standpoint, in conventional reductive methods, the modified electrodes have been achieved using chemical probase (such as triethylammonium), toxically organic solvent (such as DMF), high overpotential and high temperature.46-48,51 In contrast, EASA allows for the manipulation of various type of mMOFTFs in premeditated phase, with emphasis on adjustable porosity, orientation and morphology in mild and green conditions. This work, introduces a novel and controllable method for the fabrication of mMOFTFs with hollow 3D hexagonal packed crystals consisted of microcavity and 2D honeycomb-like mesopores, which grow perpendicularly on the two sides of electrode surface, without need to pretreatment or chemical modification. EXPERIMENTAL SECTION Chemical and Materials. Trimesic acid (H3btc, Merck, 95%), cetyltrimethylammonium bromide (CTAB, SigmaAldrich, 95%) were reagent-grade materials. Zinc nitrate hexahydrate (Zn (NO3)2 . 6H2O, Sigma-Aldrich, 98%), potassium nitrate (K NO3, Sigma-Aldrich, 99%), hydrochloric acid (HCl, Merck, 37%), ethanol (C2H5OH, Merck, 99%), sodium bicarbonate (NaHCO3, Merck 98%), sodium carbonate (Na2CO3, Merck 98%), sodium acetat

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(CH3COONa, Sigma-Aldrich, 99%), acetic acid (CH3COOH, Sigma-Aldrich, 99%), sodium hydrogen phosphate (Na2HPO4, Sigma-Aldrich, 99%), sodium dihydrogen phosphate (NaH2PO4, Sigma-Aldrich, 99%), sodium hydroxide (NaOH, Merck, 99%) were of proanalysis grade and used as received without further purification. All aqueous solutions were prepared daily with high purity distillated water from a Millipore Milli-Q water purification system at room temperature. Also, the buffered solutions were prepared based on Kolthoff tables.59 Electrochemical Deposition Setup. Typical voltammetry experiments were carried out on a classical threeelectrode undivided cell configuration with a platinum auxiliary electrode and a glassy carbon (GC) working electrode (surface area = 3.14 mm2) versus a SCE reference electrode using an Autolab PGSTAT-101 monitored by the Electrochemical System Software (Eco Chemie). Synthesis and deposition of mMOFTFs were performed in a homemade undivided three electrode cell consisting of a cap glass bottle containing a precursor solution, in which the working electrodes were glassy carbon, carbon plate (20*10*3 mm) or indium titanium oxide (ITO) plate and the auxiliary electrode consisted of a U-shaped stainless steel sheet. The working electrode potentials were measured versus a double-junction SCE reference electrode at all of the electrochemical experiments at room temperature. In order to removal of oxygen from the solution, nitrogen gas with a purity of 99.999% was used for 5 min. Initial sol preparation: In a typical synthesis, 1.33 g (4.5 mmol) of zinc nitrate as a cation source and 0.127 g sodium nitrate (0.1 M) as a supporting electrolyte were dissolved in 15 ml of deionized water (solution A). Also, 0.525 g (2.5 mmol) of trimesic acid (H3btc) and 1 g (2.7 mmol) cetyltrimethylammonium bromide (CTAB) dissolved in 15 ml of ethanol (solution B). Then, solution B was added to solution A under vigorous stirring. The precursor solution aged under stirring for 2.5 h at pH=2.1 at room temperature before the electrodeposition process. Synthesis and deposition of Zn-mMOFTF. (Modified electrode) After preparation of precursor solution, working electrodes immersed in this solution, and simultaneous synthesis and deposition of porous metal-organic framework thin film was accomplished by applying a suitable cathodic current (chronopotentiometry) or potential (chronoamperometry) for a period. The modified electrodes were swiftly taken away from the solution and immediately soaked with distilled water. The electrodeposited surfactant template film was then aged overnight at room temperature. Extraction of the surfactant template was performed in ethanol/water solution under moderate stirring for 10 min to achieve the modified electrodes with ZnmMOF thin films. Instrumentation. The film characterizations were performed using the following instruments: field emission scanning electron microscopy (FE-SEM) images were recorded using a HITACHI S-4160 and Philips CM30 microscope at an acceleration voltage of 300 kV for highresolution transmission electron microscopy (HR-TEM). ECS 4010 CHNSO Analyzer for elemental analysis (CHN),


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ICP MS ELAN DRC-e for induced coupled plasma analysis, TGA-PL 1500 for thermal gravimetric analysis (TGA). Powder X-ray diffraction (PXRD) patterns were recorded on a APD 2000 italstructures instrument in Bragg−Brentano mode (2θ−θ geometry; Cu Kα1) using a linear position sensitive detector (SAINT-GOBain), with a step width of 0.1° 2θ and scan rate = 1520 s per step (2θ = 0°−40°). Background corrected XRD patterns were normalized before plotting. FTIR spectra were recorded on a Pekin-Elmer GX FT-IR spectrometer for Fourier transform infrared spectroscopy. The samples were scratched on the surface of the electrode before IR, XRD, TEM, CHN and TGA analysis. Fluorescence and UV-visible measurements were recorded by Perkin Elmer LS50B luminescence spectrometer and SPECORD 210 analytikjena (Win ASPECT software) respectively. The impedance measurements were recorded on a Zahner Zennium (messsystem PP201) instrument, at open circuit potential with 10 mV AC modulation amplitude in the frequency range of 50 kHz to 50 mHz. A Metrohm pH meter 691 was utilized for measuring the pH of the solutions. Voltammetric measurements were performed at room temperature using an Autolab PGSTAT-101 monitored by the electrochemical system software (Eco Chemie). All the electrodes were manufactured by AZAR Electrode Company, Urmia, Iran. RESULT AND DISCUSSION In this work, we employed the EASA technique for the fabrication of a thin layer of mesoporous MOF at the electrode surface. As shown in Scheme 1 (also see Scheme S1) our procedure involves immersing a C, GC or ITO electrode in a solution containing trimesic acid (H3btc) as a ligand, zinc nitrate as a cation source and cetyltrimethylammonium bromide (CTAB) surfactant as a structure-directing agent (SDA), in the presence of potassium nitrate as a supporting electrolyte. In-situ electrogeneration of hydroxide ions at the cathode surface, by the applied current or potential, is an essential requirement in this method (NO3- reduction maybe contribute).47,48,52,53 Upon the rising of the local pH at the cathode surface, hydroxide ions cause activate of the neutral ligands (deprotonation), self-assembly of liquid crystal phase and consequently crystallizing of mMOF on the electrode surface. The prominent features of this process from the eco-friendly and green chemistry standpoint that makes progress by co-operative mechanism,5-7,25 are the synthesis in aqueous solution at room temperature, without need to any ex-situ base/probase48 or co-

template.29,30 It is noteworthy, the fine interaction of the ligand and the surfactant is an essential key in the cooperative mechanism, which lead to the fabrication of mesophase intermediates, along with growth and crystallization of MOF.30,60,61 In this regard, as imagined in Scheme 2, electrostatic attraction of negatively charged ligands (L-) and positively charged cationic surfactant (S+) lead to mesophase configuration on the electrode surface. On the other hand, coordination of electron donor ligand with electron acceptor inorganic cation, manage crystallization process and growth of MOF. It should be highlighted that, in the case of deprotonation of ligand by ex-situ base addition and before utilizing of potential, surfactant phase will be excluded.47 Under these conditions, due to the lack of cooperation mechanism, the reaction of ligand and metal ion leads to the formation of microporous MOF. Unlike this case, under electrochemical deprotonation conditions, due to the gradual electrogeneration of OH- ion and regular deprotonation of ligand, the crystallization rate between ligand and metal ion is low, so that the possibility of forming micelle phase and thus the configuration of mesoporous MOF is provided. So, electro-controllable of in-situ pH is highlighted and ruled out the use of co-template.27,29,30 It should be noted that, the coordination of a typical ligand (H3btc) with a metal cation in the absence of surfactant leads to microporous MOF.62 On the other hand, the configuration of the same mesoporous MOF using chemical methods has been performed in the presence of surfactant, with the differences that the synthesized MOF is in the powder form and needs for high temperature, pressure/vacuum and base for deprotonation, chelating agent for control of coordination rate and co-template for fine interaction.27,29,30 In this work, water has a dual role as a solvent and OH- source. In connection with the second role, it should be noted that, pH varies only in the electrode double layer and bulk solution remains unchanged. This is a key factor for control of ligand deprotonation and crystallization rate directed toward desirable phase on the electrode surface by surfactant as a structure directing agent. In chemical methods, ligand deprotonation, nucleation and crystal growth occur indiscriminately in the bulk solution, without any control on kinetic, morphology and orientation of crystallization. Furthermore, the employment of water as a protic solvent prevents the metal plating on the auxiliary electrode, owning to sufficient hydrogen evolution.43

Scheme 1: Ideal proposed scheme for the fabrication of the zinc-mesoporous metal organic framework modified electrode (Zn-


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mMOFME).

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CTAB, H3btc and scratched Zn-mMOFTF. Three important changes can be deduced from IR spectrum of ZnmMOFTF

Scheme 2: Proposed scheme for Zn-mMOF synthesis via cooperative mechanism of metal ions, ligands and surfactants before (left) and after (right) extraction of template. In order to study of the permeability of deposited mesostructure film, the cyclic voltammetry of the Fe(CN)63- redox probe was employed before and after template extraction. According to Figure 1a, both anodic and cathodic peaks of hexacyanoferrate on the bare GC electrode (Figure 1e) disappeared when, surfactanttemplated mMOFME was used, which represent the covering of the electrode surface with deposited film. The permeability of the electrode (Zn-mMOFTF) was increased and comparable with the typical response of the probe on the bare GC electrode after gradual removing of surfactant (Figure 1b- d). In addition, during the removing of surfactant, Epc and Epa shift to less negative and less positive values, respectively, which mean lower resistance of the film to diffusion of ions probe through the mesopores. These data are initial satisfying evidences for the electron transfer and diffuse of the ions probe from the bulk solution through the well-defined mesoporous MOF crystals. It may be noted that the probable limited mass transfer due to the presence of the film is greatly addressed by the porosity and Lewis acid property of the MOF film toward the negative ions probe. This case is comparable with the inside surface charge of mesoporous silica channels.52,54-57 The stability of the MOF film after cyclic voltammetry experiments was tested by FTIR, XRD and impedance techniques (See Figure S1-S3). Figure 2 shows the cyclic voltammograms of Zn-mMOFTF modified electrode in acidic (acetate buffer, pH= 4.0) an basic (bicarbonate buffer, pH = 9.0) blank aqueous solution. The comparison of these CVs with that of zinc nitrate CV (Figure 2, inset) confirms the presence of active zinc ion in the lattice of deposited crystals. To study of the functionality and bonding properties of the deposited film on the electrode surface, IR analysis of scratched film was done. Figure 3 shows IR spectra of

Figure 1: The cyclic voltammograms of Fe(CN6)3- (1.0 mM) at the Zn-mMOFME; (a) before and (b, c, d) after continuous extraction of surfactant. (e) Cyclic voltammogram of Fe(CN)63-(1.0 mM) on the bare GC electrode. Supporting electrolyte: 0.15 M KNO3. Scan rate: 100 mV s1 .

Figure 2: The electrochemical behavior of Zn-mMOFME in -1 blank solution with scan rate of 100 mV s ; (a) bare carbon electrode. (b) Zn-mMOFME in a basic (bicarbonate buffer, pH = 9.0) and (c) in an acidic (acetate buffer, pH = 4.0) solution. Insert: cyclic voltammogram of Zn(NO3)2 (0.01 M) at bare carbon electrode. over the spectra of the raw materials.26,60,61 First, the absence of peaks corresponding to carboxylic acid function (3085-2554 cm-1) in the deposited film which reveals the contribution of the mentioned group in the formation of MOF. Second, the strong couple bands at 1626-1578 and 1443-1375 cm-1 can be assigned to the asymmetric and symmetric vibration of carboxylate anions. These bands in the free COO- appear at 1721-1607 and 1455-1404 cm-1. This is an additional evidence of the coordination of organic ligand with a metal ion. Third, the presence of the asymmetric stretching and symmetric bands, 2920 and 2850 cm-1 respectively in Zn-mMOF spectrum which is


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related to the CH2 groups of the CTAB alkyl chains and show co-operative of surfactant in electro-crystallization of ligand and metal ion. It should be noted that, all synthesized Zn-mMOFTF in the potential range of -1 to -1.5 V show the same pattern (See Figure S4 – S7).

Figure 3: The FT-IR spectra of H3btc, CTAB and scratched Zn-mMOFTF before extraction of template.

In the following, the mesoporosity and crystallinity of the scratched Zn-mMOFTFs were characterized by ex-situ powder X-ray diffraction (Figure 4). The low-angle PXRD pattern presents a single d100 peak that suggests a wormhole-like mesostructure.29 The well-defined crystallinity of the framework was also proved by the wide angle PXRD pattern and compared with the simulated pattern.62 This result is in contrast with the previously published data on the synthesized mMOF by chemical methods, which shows an amorphous structure without any XRD peaks at high angles.26,60 The calculated crystal size of scratched mMOF is approximately about 80 nm, by applying of Scherrer equation on the sharpness of the peaks. In addition, this pattern strongly rejects zinc plating on the electrode surface during the electroreduction of the precursor.47 (See Figure S8) This unexpected result reasoning to increased stability of zinc ion directed to exclusive crystallization of mMOF, in the presence of a suitable concentration of the multidentate ligand along with sufficient stream of hydrogen evolution on the proximity of electrode, stemming from electroreduction of huge amount accessible water.

Figure 4: The ex-situ powder X-ray diffraction survey of scratched Zn-mMOFTF: (I) low-angle pattern and (II) wide angle pattern.

Furthermore, the N2 adsorption isotherm with a hysteresis

loop (between 0.4-0.8 p/p0) is characteristic of a “Type IV” isotherm that is typical of mesoporous material.22,24,29,30 Also, pore size distribution calculated from Barret-Joyner-Halenda (BJH) method shows the average pores size with diameter of 3.28 nm (rp= 1.64 nm). In addition, the Brunauer–Emmett–Teller (BET) specific surface area (SBET) measured from N2 isotherms is 224 m2 g-1 that is consistent with the fact that the total surface area can be extremely small for the thin porous films compared with bulky materials. (See Figure S9) The induced couple plasma (ICP) technique and elemental analysis were used to calculate the amount of Zn and other elements in the digested Zn-mMOFTF after extraction of surfactant. As Table 1 shows, the amount of zinc ion is 22.99 wt. % that is approximately close to the theoretical value (23.99 wt. %). In addition, the amount of nitrogen is 0.12 wt. % that can be related to either residual of surfactant or nitrate ion in the pores.26 Furthermore, the amount of carbon (28.35 wt. %) is less than expected value (29.81 wt. %). This point, along with the amount of hydrogen (3.64 wt. %) led us to consider 5 water molecules into the pores of the MOF. According to the present data, the calculated amount of zinc ion in the mesostructure is 18.03 wt. % and the molar ratio of ligand to metal is approximately 1:1. Table 1: ICP and CHN data of scratched Zn-mMOFTF.

The thermal gravimetric (TGA) and differential thermogravimetric analysis (DTA) of scratched film, under an air atmospheric environment are shown in Figure 5. There are three distinguished weight loss steps in the temperature ranges 50–170 and 400–500 °C.Two weight losses around 100 0C are assigned to the removal (evaporation) of ethanol and water molecules, respectively. However, the predominant fragmentation zone is related to the decomposition of organic ligand, collapse and degradation of framework of mMOF in the temperature ranges 400-500 0C. The TGA results also showed the residual amount (30 % by weight) corresponds to the formation of ZnO in high temperatures,26 which is satisfyingly consistent with the results obtained by CHN and ICP analysis. These data also confirm the thermal stability of ZnmMOFME up to 400 C0 under air flow, so that the presence of mesophase does not subvert the thermal stability of the structure. Another important issue that needs to be considered is the crystal orientation and morphology, which are efficient factors in diffusion and mass transfer, especially in electrochemical studies. In this part, the images of field


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emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), revealed the alignment of 3D hexagonal crystals mainly oriented perpendicularly to the electrode surface. The unique morphology without any impurity of images is consistent with the PXRD pattern.

Figure 5: (a) TGA and (b) DTS curves of the scratched ZnmMOFTF.

To evaluate the effects of electrochemical operational variables on the size, thickness and morphology of the crystalline thin films, the effect of applied current density and potential were studied. Figure 6 (a, b, c) shows, FESEM images of synthesized Zn-mMOFTF under galvanostatic conditions (3 mA cm-2). As can be seen, one of the most significant advantages of this method is full coverage of the electrode surface and in-situ complete repairing of the cracks. Interestingly, in contrast with the previous report,44 in this work, both sides of the electrode surface are covered with MOF crystalline thin film. As a worthwhile note, this procedure avoids the “piling up” of the crystals on the top of formerly grown crystals, because in this method, pro-

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longing the electrolysis time or increasing current density is directly toward the growth and increasing the size and forming of initial crystals. Figure 6 shows right angle of crystals to the electrode surface that the appeared detour angle for some of crystals can be caused by the tensions of growing 3D crystals in a limited space. The galvanostatic method by identical conditions, can be used for various types of electrodes, without need to know the details of overpotential which in controlled-potential method is required. Nevertheless, we also used the controlledpotential mode to obtain further data on the synthesis and depositional features. Figure 7 (a, b, c) shows, FE-SEM images of synthesized Zn-mMOFTF under controlled-potential conditions (Eapp = -1.3 V vs. SCE). Similar to the galvanostatic method, this Figure shows, almost perpendicularly oriented 3D hexagonal thin film on two sides of the electrode. At constant electrolysis time (10800 s), with changing potential from 1.3 to -1.0 V, the nucleation rate decrease which results in the formation of crystals in the needle and belt-like forms (Figure 7b and S10). In contrast, at applied potential of 1.3 V, with prolonging electrolysis time from 1800 to 10800 s, electrogeneration of hydroxide ion is its highest level (high nucleation rate) and the generated crystals have enough time for growth in premeditated form (3D hexagonal packed)(Figure 7c and S11). It should be noted that, at more negative applied potentials (Eapp < -1.3 V) or high applied current densities (Iapp > 3 mA cm-2), due to the high generation rate of hydroxide ion, the amorphous materials are formed (see Figure S12) and the risk of zinc deposition increases on the other hand. Conversely, when Eapp is less negative than -1.0 V or Iapp < 1 mA cm-2, the electrogeneration rate of hydroxide ions is low, and consequently, the crystallization rate is slow and need to very prolonged time at room temperature. It should be noted that, the longer deposition time, leads to a thicker mMOF thin film and undesired aggregates (See Figure S13-14).


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Figure 6: (a, b, c) Large and close view FE-SEM and (d, e, f) HR-TEM images of scratched Zn-mMOFTF at the Iapp = 3 mA cm-2 and different times and levels of magnification. Electrolysis time in a, c, d, e and f is 10800 s and for b is 300 s.

Figure 7: (a, b, c) Large and close view FE-SEM and (d, e, f) HR-TEM images of scratched Zn-mMOFTF at the Eapp= -1.3 V vs. SCE and different times and levels of magnification. Electrolysis time in a, c, d, e and f is 10800 s and for b is 300 s. In addition to Figures 6 and 7, Figure 8 shows the ZnmMOFTF 3D crystals with exclusive microcavity which established the potential of this material as an absorbent in heterogeneous systems. Finally, despite the sensitivity of MOFs to the electron beam,24,26,29 Figures 6 and 7 (d, e,

f) revealing a highly porous well-defined 2D hexagonal mesostructure in the wall of the framework. As a conclusion, Zn-mMOFTFs consisted of the hollow 3D hexagonal microcrystals which vertically aligned on the electrode surface. The thickness of the wall of the microcrystals is


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found to be about 200 nm which composed of a welldefined 2D honeycomb-like (hexagonal) mesopores with a diameter of about 3 nm. So, the relation between the outer and internal space of cavity, which is a huge part of microcrystal with a diameter of nearly 1 micron, will be connected through the fine mesoporous wall. On the other word, the mesoporous phase has formed within the whole microporous structure of the parent MOF. The presence of surfactant as a structure directing agent has two important effects: (a) it led to the configuration of 3D hexagonal morphology and (b) it also led to the construction of 2D mesoporous structure without any negative effect on the origin MOF structure. In addition, the impedance spectroscopy, UV and fluorescence techniques have been used for further study of ZnmMOFTF modified electrode. To achieve this purpose, we used indium tin oxide (ITO) coated glass as the electrode. Figure 9 (I and II) exhibits the impedance response of bare ITO and Zn-mMOFTF/ITO electrodes in the presence of Fe(CN)63-/4- redox probe, respectively. The comparison of

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negatively charged samples.

Figure 9: Nyquist plots of bare ITO (I) and Zn3-/4-

mMOFTF/ITO plate (II) for Fe(CN)6 redox probe. UVvisible (III) and fluorescence (IV) spectra of Zn-mMOFTF deposited on ITO plate (a: Bare ITO and b: ZnmMOFTF/ITO)

CONCLUSION

Figure 8: Closer view FE-SEM images of the Zn-mMOFTF -2

deposited at Iapp = -2 mA cm and T= 5400 s on the GR electrode

the Nyquist plots of bare (curve I) and Zn-mMOFTF/ITO modified (curve II) electrodes shows that, the ZnmMOFTF as a modifier facilitates the electron transfer kinetic. In other words, the closer inspection reveals that the Zn-mMOFME layer accelerates the mass transfer (comparison of 20 Hz in curve II with 0.74 Hz in curve I). This can be related to comfortable diffusion through the mesoporous framework by the favorable electrostatic interaction between the negatively charged probe (Fe(CN)63-/4-) and the positively charged sites on the lattice of the film. Figure 9 (curve III) illustrates the absorption spectrum of Zn-mMOFTF/ITO modified electrode in the air and Figure 9 (curve IV) shows the fluorescence spectrum of the electrode in the aqueous solution. Based on these preliminary results, the Zn-mMOFTF modified electrode may be regarded as an electrochemical sensing device. The mesoporosity, large cavities and the presence of positively charged sites in the Zn-mMOFTF modified electrode makes it possible for the determination of gaseous and

This research provides a general, eco-friendly and simple electrochemical procedure for the fabrication of mesoporous MOF thin Films. In this method, the electrochemically driven co-operative reaction of SDA, ligand and cation in water as a green solvent, leads to the in-situ and simultaneously synthesis and deposition of mMOFTF onto the desired conductive surface. The EASA technique allows us to achieve perpendicularly aligned 3D hollow hexagonal microcrystals onto the electrode surface, with 2D hierarchical honeycomb-like mesopores in the wall of cavities. The faster electron transfer kinetics and improved diffusion of analysts along with, suitable surface area and permeable to external reagents, makes the fabricated mMOFTFs as a potential entrant in sensing applications. From the standpoint of environmental and economic concerns, this method makes use of water as both a solvent and a reactant, without recourse to any chemical agent such as a base or probase. Other prominent features of this work are the use of electricity instead of temperature and vacuum/pressure and the short reaction time (300 seconds) instead of prolonged reaction time (several days). Furthermore, besides the substrate flexibility (C, GC, ITO), the protocol does not require any pretreatment, activation or chemical modification of the underlying surface. Authors profoundly believes that, the employment of the EASA method for the synthesis of mMOF materials (for the first time), along with consideration of the above-mentioned advantages can make a good foundation and new challenging outlook for the modification and functionalization of novel types of electrodes and sensing devises and heterogeneous systems with mesoporous MOF thin films. The extension of this methodology to the fabrication of new types of mMOF


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thin films (modified electrodes) via EASA technique and their applications is ongoing in our research group.

ASSOCIATED CONTENT Further data of IR, XRD and FE-SEM analysis can be found in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] and [email protected] Author Contributions All authors have given approval to the final version of the manuscript.

Funding Sources The Authors declare no competing for financial interests.

ACKNOWLEDGMENT The authors are grateful to the BASU research council. We also appreciate Dr. Hamed Moghanni for his comments on the impedance tests.

ABBREVIATIONS mMOF, Mesoporous Metal-Organic Framework; microMOF, microporous MOF; mMOFTF, mMOF thin film; mMOFME, mMOF modified electrode; EASA, Electrochemically Assisted Self-Assembly; FE-SEM, Field Emission Scanning Electron Microscopy; HR-TEM, High Resolution Transmission Electron Microscopy; CHN, Elemental Analysis; ICP, Induced Coupled Plasma; TGA, Thermal Gravimetric Analysis; PXRD, Powder X-ray Diffraction; FT-IR, Fourier Transform Infrared Spectroscopy; GC, Glassy Carbon; ITO, Indium Titanium Oxide; SCE, Saturated Calomel Electrode REFERENCES (1) Kitagawa, S.; Kitaura, R.; Noro, S. i. Angew. Chem. Int. Ed. 2004, 43, 2334-2375. (2) Férey, G. Chem. Soc. Rev. 2008, 37, 191-214. (3) Silva, P.; Vilela, S. M.; Tomé, J. P.; Paz, F. A. A. Chem. Soc. Rev. 2015, 44, 6774-6803. (4) Xu, Q., Nanoporous Materials: Synthesis and Applications. CRC Press, Boca Raton: 2013. (5) Yang, P., The chemistry of nanostructured materials. World Scientific, New Jersey, London: 2003. (6) Li, W.; Zhao, D. Chem. Commun. 2013, 49, 943-946. (7) Hoffmann, F.; Cornelius, M.; Morell, J.; Fröba, M. Angew. Chem. Int. Ed. 2006, 45, 3216-3251. (8) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. Science 2013, 341, 1230444. (9) Li, H.; Eddaoudi, M.; O'Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276-279. (10) Furukawa, H.; Müller, U.; Yaghi, O. M. Angew. Chem. Int. Ed. 2015, 54, 3417-3430. (11) Czaja, A. U.; Trukhan, N.; Müller, U. Chem. Soc. Rev. 2009, 38, 1284-1293. (12) Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; SchierleArndt, K.; Pastre, J. Journal of Materials Chemistry 2006, 16, 626636. (13) Muller, U.; Putter, H.; Hesse, M.; Schubert, M.; Wessel, H.; Huff, J.; Guzmann, M., Method for Electrochemical

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76x44mm (300 x 300 DPI)


Electrochemically Assisted Self-Assembly Technique for the

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