Rapid, Selective, Ambient Growth and Optimization of Copper

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Rapid, Selective, Ambient Growth and Optimization of Copper Benzene-1,3,5-Tricarboxylate (Cu-BTC) MetalOrganic Framework Thin Films on a Conductive Metal Oxide Scott E Crawford, Ki-Joong Kim, Yang Yu, and Paul R Ohodnicki Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00016 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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Crystal Growth & Design

Rapid, Selective, Ambient Growth and Optimization of Copper Benzene-1,3,5-Tricarboxylate (Cu-BTC) Metal-Organic Framework Thin Films on a Conductive Metal Oxide Scott E. Crawford,a‡ Ki-Joong Kimab‡*, Yang Yu,ab and Paul R. Ohodnicki,ac a

National Energy Technology Laboratory, 626 Cochrans Mill Rd., Pittsburgh, Pennsylvania

15236, United States b

c

AECOM, 626 Cochrans Mill Rd., Pittsburgh, Pennsylvania 15236, United States

Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh,

Pennsylvania 15216, United States

KEYWORDS: copper benzene-1,3,5-tricarboxylate (Cu-BTC), metal organic framework (MOF) thin films, conductive metal oxide, hydroxy double salts.

ACS Paragon Plus Environment

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ABSTRACT: Metal organic frameworks (MOFs) are useful for thin film-based device integration as they show high adsorption capacity for selected gases. However, optimized device integration of MOFs for various platforms requires high-quality, well controlled thin film growth techniques that are flexible, scalable, and manufacturable. For this reason, there is a critical need for the ability to grow MOFs in the form of dense and uniform thin films efficiently on a wide range of substrates. In this work, copper benzene-1,3,5-tricarboxylate (Cu-BTC) MOF thin films are rapidly produced at room temperature by using a conductive aluminum-doped zinc oxide (AZO) as a seed layer to template Cu-BTC growth, with growth only occurring on the AZO layer via a hydroxy double salt intermediate. The formation pathway of the Cu-BTC films was investigated in detail because of the significant importance of improving the Cu-BTC film growth from the perspective of optimized device integration. We demonstrate that the structure of the resultant Cu-BTC film could be fine-tuned via alterations to the solvents used during growth conditions, pH, and the identity of the Cu salt anion. The technique described here is rapid, tunable, selective, and applicable to a variety of substrates.

INTRODUCTION Metal-organic framework (MOF) thin films are an attractive candidate material for applications in luminescence, chemical sensing, catalysis, membranes and optical device fabrication due to their thermal stability (up to several hundred degrees Celsius), high surface area, and tunable pore properties.1-5 MOFs are highly porous materials consisting only of metal ions and organic linkers, which combine to form highly ordered, porous crystals with high surface area. Parameters such as MOF reactivity and pore size may be controlled through careful


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selection of the metal ions and organic linkers, promoting selectivity towards specific gas molecules. A rich variety of MOF structures may be accessed due to the seemingly endless number of potential metal atom and organic linker combinations.5 Copper benzene-1,3,5tricarboxylate, specifically Cu3(BTC)2 • 3H2O phase (Cu-BTC) is regarded as a particularly promising material with a maximum gas uptake and selectivity for CO26-8 and CH4,6,9,10 two critical gasses within the energy sector due to their implications for energy production and the climate. A critical step towards the development of MOF thin film devices is the ability to efficiently and reliably incorporate high-quality MOFs onto a range of substrates, such as optical fibers2,11,12 or piezoelectric crystals13-16. There are several techniques that have been established for MOF deposition. Typically, the substrate is modified with a “self-assembly” layer, such as an organic linking molecular or metal oxide film, to which the MOF will adhere during crystal growth. Following the substrate surface modification, a MOF layer may be grown using layer-by-layer deposition,17-19 a polymer template,20,21 solvothermal deposition,13,22-24 electrochemistry,25 and/or dip-coating.26 However, these techniques are often inconvenient due to either long reaction times, heating requirements, equipment costs, and/or poor control over crystal coverage and morphology. A promising alternative method is to use the metal oxide itself as a template for MOF growth by sequential exposure to the metal cation and then the organic linker.27 This technique has been developed to grow Cu-BTC MOFs rapidly and at room temperature.28 Using a metal oxide film to template Cu-BTC growth circumvents the long reaction times, equipment costs, and heating steps required by other methods. Because MOF growth will occur only on the metal oxide, waste from side reactions will be mitigated, providing a more environmentallyfriendly process compared to conventional techniques. Further, this method should be applicable


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to any substrate that is amenable to metal oxide deposition. The formation pathway of the MOF films through the described growth technique has not been investigated in full detail but is critical to improving diverse MOF film growth for optimized device integration. Therefore, optimization of this process and a thorough mechanistic understanding are needed to produce Cu-BTC films of sufficient quality for applications, as precise control over MOF crystal size and packing density is required. Here we report the rapid, selective, and room temperature growth of Cu-BTC thin films on aluminum-doped zinc oxide (AZO) templates. AZO is a transparent, conductive material that has been exploited for a wide range of applications including photovoltaics,28-30 plasmonics,31,32 catalysis,33 and sensing,34,35 and is hence an intriguing candidate to serve as a template for MOFbased devices. In this work, the AZO is first exposed to different Cu salt ions, followed by incubation in a BTC solution, leading to rapid growth of Cu-BTC only on the AZO layer at room temperature. Analyses of each reaction step indicates that a hydroxy double salt (HDS) intermediate is formed, which has structural properties that can be tuned via alterations to the Cu salt anion and pH. The concentration and identity of each solvent also influences Cu-BTC growth. Taken together, these experiments provide an efficient method for depositing Cu-BTC thin films while also providing new insights into the synthetic variables that may be used to finetune the final MOF structure for specific applications, a key step in device integration.

EXPERIMENTAL SECTION

Materials and Methods


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Reagent grade of copper (II) nitrate (Cu(NO3)2•6H2O), copper (II) sulfate (Cu(SO4)2•5H2O), copper (II) acetate (Cu(CHCOO)2), copper (II) chloride (CuCl2•2H2O), copper (II) bromide (CuBr2), aluminum nitrate nonahydrate (Al(NO3)3•9H2O), zinc acetate (Zn(CHCOO)2•2H2O), ethanolamine (EA), dimethylsulfoxide (DMSO), dimethylformamide (DMF), isopropyl alcohol (IPA), ethanol (EtOH), methanol (MeOH), acetone (ACT), acetonitrile (ACN), sodium hydroxide (NaOH), hydrochloric acid (HCl, 37%), and benzene-1,3,5-tricarboxylic acid (BTC) were purchased from Sigma Aldrich (St. Louis, MO). Soda lime glass slides were obtained from Thermo Fisher Scientific (Waltham, MA). All chemicals were used as purchased without further purification.

Preparation of AZO layers Glass slides were cut into three square sections and were sonicated for 5 minutes in ACT, then MeOH, and then deionized water, followed by storage in EtOH until usage. An AZO spincoating technique was used and modified from the method proposed by Ryzdek and coworkers.36 Briefly, a precursor solution was prepared consisting of 0.018 M Al(NO3)3•9H2O, 0.6 M Zn(CHCOO)2, and 0.6 M EA and 20 mL IPA. The solution was heated and stirred for 2 hours at 60 oC. The precursor solution was added dropwise to the glass substrate and was spin-coated at 3000 rpm for 30 seconds, which was then dried for 30 seconds at 100 oC in air. This cycle was typically repeated three times, followed by annealing for 2 hours at 550 oC in air.

Growth of Cu-BTC films In a typical deposition, AZO-coated substrates were dipped into the Cu salt solutions for 1 minute, and rinsed with deionized water and dried under nitrogen. The substrates were then


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dipped into a solution containing H2O, EtOH (containing BTC), and/or a polar aprotic solvent (DMSO, DMF, ACT, or ACN) for 15 minutes with drying under nitrogen. Conditions varied depending upon the experimental variable being analyzed (see below). For comparison, bulk CuBTC powder was synthesized by a typical solvothermal method in the batch reactor.37 A solution of 3.6 mM Cu(NO3)2•6H2O in 60 mL of solvent consisting of equal parts EtOH and H2O and 8.1 mM BTC in 60 mL of EtOH were placed into a Teflon autoclave, then heated to 120 °C, and held for 18 hours in a convective oven. The product was then cooled, centrifuged/washed with EtOH several times, and dried under vacuum for 24 hours at 70 °C.

Copper anion experiments To determine how the Cu anion influence MOF growth and morphology, aqueous, 0.29 M solutions of Cu(NO3)2, CuCl2, CuBr2, Cu(CH3COO)2, and Cu(SO4)2 were prepared. AZO-coated substrates were exposed to the different Cu salt solutions for 1 minute, rinsed with deionized water, and dried under nitrogen. The substrates were then dipped into a 1:1:1 ratio (9 mL total volume) of DMF:H2O:EtOH (0.17 M BTC) for 15 minutes, followed by nitrogen drying.

Acid-base experiments To determine the influence of acid and base addition on Cu-BTC growth and morphology, AZOcoated substrates were first dipped for 1 minute into a 0.29 M Cu(NO3)2 solution, rinsed in deionized water, and dried under nitrogen. A solution originally containing a 1:1:1 ratio (9 mL total volume) of DMF:H2O:EtOH (0.17 M BTC) was prepared, and additional volumes (ranging from 10 to 1500 µL) of aqueous 0.1 M NaOH or HCl were added. The AZO substrates were exposed to the resultant growth solution for 15 minutes, followed by nitrogen drying.


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Solvent Experiments To determine the influence of both the polar aprotic solvent identity and concentration, AZOcoated substrates were first dipped for 1 minute into a 0.29 M Cu(NO3)2 solution, rinsed in deionized water, and dried under nitrogen. Growth solutions were prepared containing 0 to 6 mL of either DMSO, DMF, ACN, or ACT, 3 mL of EtOH containing 0.17 M BTC, and enough H2O to bring the total volume to 9 mL. To determine whether water is needed for Cu-BTC growth, a solution containing 5 mL of DMF, 3 mL of 0.17 M BTC in EtOH, and 1 mL of EtOH was used. In all cases, the substrates were dipped into the growth solution for 15 minutes, followed by nitrogen drying.

Characterization Transmission and absorption spectra for all films were collected using a Perkin-Elmer Lambda 1050 spectrophotometer equipped with an integrating sphere to include non-specular light associated with diffuse scattering. Symmetric theta-2theta X-ray diffraction (XRD) scans were performed using a Panalytical X’Pert Pro MRD X-ray Diffractometer, operating at 40 kV and 40 mA with Cu kα radiation (0.154 nm). Scanning electron microscope (SEM) analyses were conducted with an FEI Quanta 600 using a 20 kV accelerating voltage.

Results and Discussion

Confirmation of a hydroxy double salt intermediate


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The formation of a hydroxy double salt (HDS) is an anticipated intermediate step in the growth of Cu-BTC on metal oxide substrates.27 To gain insight into the Cu-BTC growth mechanism, SEM and transmission spectroscopy techniques were used to characterize each step of the Cu-BTC growth process: the AZO thin film, the AZO thin film following Cu2+ exposure, and the resultant Cu-BTC thin film produced following exposure to BTC (Figure 1A). SEM images shows that the AZO layer composed of spherical nanoparticles was uniformly prepared by the spin-coating method with a thickness of ~110 nm on the substrate. Following AZO exposure to Cu(NO3)2 for 1 minute, 2D plate-like crystals are observed, which is similar to reported HDS structures formed with Cu(NO3)2 and ZnO.38 Octahedral particles are formed by exposing the HDS to a BTC solution for 15 minutes. Transmission spectroscopy indicates an increase in the transparency following exposure to Cu2+ which is associated with changes in film thickness and refractive index as well as the degree of scattering due to the rough film morphology. Following BTC exposure, a decrease in transmission is observed at ~700 nm, which is associated to d-d band transitions of the Cu ion, indicative of Cu-BTC formation (Figure 1B).39 XRD patterns show the formation of (Zn, Cu) hydroxy nitrate as a HDS intermediate40,41 and Cu-BTC following exposure to BTC (Figure 1C). In addition, Cu-BTC film with preferred growth along the (111) direction with complete surface coverage was obtained. This is likely associated with the vertically grown 2D structure crystals of the HDS intermediate. Taken together, these experiments indicate that the formation of the HDS intermediate is a key step towards the formation of Cu-BTC on the AZO layer (N.B. reversing the synthesis order by exposing the AZO substrate to BTC first followed by Cu(NO3)2 does not induce Cu-BTC formation).


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Figure 1. (A) SEM images, (B) transmission spectra, and (C) XRD patterns of the AZO layer before and after Cu(NO3)2 solution exposure and the resultant Cu-BTC produced by exposure to BTC solution (1:1:1 ratio of DMF:H2O:EtOH).

Copper salt anions influences Cu-BTC morphology It has recently been demonstrated that the morphology of a Cu-based HDS is sensitive to the identity of the Cu2+ salt anion.38 Because the Cu2+ HDS is an intermediate step in the formation of Cu-BTC on AZO, it is anticipated that altering the Cu2+ HDS via the use of different anions will alter the structure of the MOF film. Parameters such as the anion size and affinity for Cu2+


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may influence this process. Here, we investigated the influence of chloride, sulfate, nitrate, acetate, and bromide anions on Cu-BTC formation. Interestingly, little to no Cu-BTC formation is observed when acetate and sulfate, the two largest anions tested, are used (Figure S1). Conversely, rapid Cu-BTC formation is observed when the smaller chloride, bromide, and nitrate anions are used (Figure 2A). Narrow size distribution of particles and higher coverage is observed when Cu(NO3)2 is used, suggesting faster nucleation and slower growth. Poor coverage and bigger particles are observed when CuCl2 and CuBr2 are used, indicating a slow nucleation and fast growth regime.42,43 These observations do not trend with salt solubility (and, by extension, the strength of Cu-anion interaction). However, there is a qualitative trend with anion hydrodynamic radii,44 and it is possible that the presence of large anions produces steric effects that inhibit Cu-BTC formation. A likelier scenario is that anion-dependent alterations to the HDS itself promote or prevent Cu-BTC formation, as the Cu salt identity is known to influence HDS structure.38 These structural differences may increase the activation energy required for MOF growth and also alter the nucleation and growth kinetics of the MOF.43 To probe the influence of the Cu salt anion on HDS morphology, SEM images were taken of the AZO-coated substrates following a 1 minute exposure time to each Cu salt. Remarkably, no significant HDS formation is observed when sulfate or acetate are used under these experimental conditions (Figure S2), whereas a 2D structured HDS (needle-like or plate-like) is formed when chloride, bromide, or nitrate are used (Figure 2A). These observations trend with the stability constants for Cu (II) with each anion; Cu(II) complexes with acetate and sulfate exhibit significantly higher stability constants than those formed with chloride, bromide, or nitrate.45,46 Hence, anions with a lower stability constant with Cu (II) are capable of forming an HDS in the presence of AZO, whereas ions with a high stability constant with Cu (II) do not form an HDS, and, by extension, Cu-BTC,


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under the experimental conditions used here. The correlation of Cu-BTC growth with HDS formation highlights the importance of the HDS intermediate as an essential component for MOF growth on metal oxide substrates. The lack of MOF growth when sulfate or acetate was used was also confirmed by measuring the absorption spectra of each film following BTC exposure (Figure 2B). For the three smaller anions, an intense absorption peak is observed at ~700 nm, whereas little to no absorption is observed in this range from the sulfate or acetate samples. The MOFs grown using the nitrate, chloride, and bromide salts were also characterized by XRD, as shown in Figure 2C. There is no indication of unreacted HDS intermediates in the Cu-BTC films following BTC exposure when chloride and nitrate salts are used, indicating complete consumption of the HDS as the Cu-BTC film forms. However, some Cu-HDS still remains following BTC exposure when CuBr2 was used, indicating that a longer exposure time to BTC solution may be required for the reaction to go to completion. In addition, undesireable Cu2OH(BTC)(H2O)n•2nH2O phase47 as evidenced by an peak at 2θ = 10.4° indicated by an asterisk is observed. Both SEM and XRD analyses of the HDS films formed using nitrate, bromide, and chloride reveal morphological differences depending on Cu salt identity, which likely contributes to the structural differences between the resultant Cu-BTC films. Taken together, these results indicate that the Cu salt anion may be used to tune Cu-BTC nucleation and growth kinetics.


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Figure 2. (A) SEM images, (B) absorption spectra, and (C) XRD patterns of the AZO layer after exposure to different Cu salt ions and Cu-BTC films produced by exposure to BTC solution (1:1:1 ratio of DMF:H2O:EtOH). An asterisk on the XRD spectrum of AZO-CuBr2-BTC indicates the Cu2OH(BTC)(H2O)n•2nH2O phase.47

Acid inhibits the growth of Cu-BTC


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Changes to the growth solution pH are anticipated to alter Cu-BTC growth kinetics through several potential mechanisms. An important consideration is the pH-dependent protonation state of the BTC molecule during growth. It has been reported that if only two carboxylic acid groups on BTC are deprotonated, a fibrous coordination polymer forms rather than a MOF crystal.24 Additionally, the HDS itself is expected to be pH sensitive because it contains hydroxide ions. Here, Cu-BTC films were grown in the presence of increasing HCl concentration (hereafter, Cu(NO3)2 was used for all tests because it shows a high surface coverage without HDS intermediates in the Cu-BTC film). Interestingly, while no clear influence of HCl is observed at low HCl concentrations, Cu-BTC growth is inhibited under increasingly acidic conditions, with no growth observed for the highest added acid amount (Figure S3). No obvious influence on CuBTC formation is observed when NaOH is added, although the BTC precipitates out of solution when more than 1.5 mL NaOH is added to the growth solution. Absorption spectra confirm a general insensitivity to basic conditions with growth inhibition under more acidic conditions (Figure 3A). To further assess the impact of acid and base addition on the final Cu-BTC morphology, SEM images were taken of the 1500 µL NaOH and the 500 µL HCl samples, which were the highest acid or base concentrations where MOF growth was observed, respectively (Figure 3B). Interestingly, homogenous films were produced in all cases, with a decrease in average MOF edge length from 710 nm for NaOH, 590 nm without acid or base, to 420 nm when HCl is added. This decrease in particle size indicates a pH-dependent growth rate, with acid acting as a growth inhibitor, leading to smaller particles. Subsequently, control over pH can be used as an additional parameter to tune the MOF morphology.


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Figure 3. (A) Absorption spectra of Cu-BTC films grown at different acid and base additions. (B) SEM images of Cu-BTC films produced by adding 1500 µL NaOH and 500 µL HCl in the BTC solution (1:1:1 ratio of DMF:H2O:EtOH).

Impact of solvent on the Cu-BTC morphology Common synthetic routes for Cu-BTC typically involve the use of alcohols (typically EtOH), H2O, and/or a polar aprotic solvent such as DMF or DMSO.21,48 The polar aprotic solvent stabilizes the solvated BTC and Cu ions, likely through the formation of both metal-solvent coordination complexes and hydrogen-bonded solvate structures with BTC.20 Additionally, the polar aprotic solvent also influences the pH of the growth solution which further tunes Cu-BTC morphology.49 To our knowledge, the use of other polar aprotic solvents such as ACT and ACN has not been reported in the literature despite their structural similarities to DMF and DMSO. Previous reports have demonstrated that altering the concentration of the polar aprotic solvent


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can influence crystallizaiton processes such as nucleation kinetics, lattice parameters, and desolvation processes, each of which influences MOF growth kinetics and structure.24,49-51 Note that for these experiments, the absence of any polar aprotic solvent (i.e. using only H2O and EtOH) produced the rapid formation of large Cu-BTC crystals that would not adhere to the glass substrate due to their large size. Hence, the presence of a polar aprotic solvent is required under the synthetic conditions reported here to control growth kinetics of Cu-BTC. Conversely, when no H2O is used, no Cu-BTC growth is observed in the timescale of these experiments. Here, Cu-BTC films were grown in the presence of different volumes of ACT, ACN, DMSO, and DMF to determine how solvent identity and concentration influence Cu-BTC morphology. For each solvent, the polar aprotic solvent volume was varied from 1 to 6 mL, with 3 mL of 0.17 M BTC in EtOH and sufficient H2O to produce a 9 mL total volume. When no H2O is present (i.e. when 6 mL of polar aprotic solvent is used), no growth is observed. The extent of growth at a given volume was dependent upon the identity of the polar aprotic solvent; for instance, little to no Cu-BTC growth was observed when only 1 mL of DMF was used, yet for ACT the strongest Cu-BTC growth was obtained with 1 mL. Representative photographs of MOF films grown using each solvent at each volume (Figures S4-S7) and the corresponding absorption spectra (Figures S8-S11) are shown in the supporting information. Interestingly, SEM (Figure 4A) characterization indicates that solvent-dependent morphological differences arise in the Cu-BTC structure depending upon the identity of the polar aprotic solvent. ACN and ACT, which are more acidic than DMSO or DMF, produce a mixture of Cu-BTC crystals as well as a fibrous network that become more prevalent at higher ACT and ACN concentrations (Figure S12). Similar structures have been reported for Cu-BTC when the BTC is not fully deprotonated.24 Hence, the acidic nature of the ACT and ACN relative to DMSO/DMF may promote this fiber-


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like secondary morphology, as the more basic DMF and DMSO yielded only Cu-BTC crystals. For each solvent type, XRD analysis indicates Cu-BTC formation, as evidenced by the emergence of the Cu-BTC (222) peak for each film. Remaining HDS intermediate were observed at ACT and ACN solvents after exposure BTC solution, indicating that a longer time may be required for the reaction to go to completion. Additional features at 2θ = 9.1°, 10.3°, and 18.0° indicated by an asterisk are observed at in the ACT solvent, which can be assigned to Cu2OH(BTC)(H2O)n•2nH2O.47 This phase is undesireable because it has a much lower surface area than Cu3(BTC)2•3H2O phase.37 These results suggest that the morphology, growth kinetics, and phase of Cu-BTC are strongly dependent on the solvent used in the growth solution (Figure 4B). To better understand how solvent volume impacts Cu-BTC film morphology, SEM analysis was conducted for MOFs grown using 3, 4, and 5 mL of DMF (Figure S12). The size of the crystals increased slightly with increasing DMF concentration, however the packing density decreases. Hence, solvent identity and concentration could also be used to influence Cu-BTC growth and morphology.


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Figure 4. (A) SEM images and (B) XRD patterns of Cu-BTC MOFs grown using 4 mL ACT, ACN, DMSO, and DMF in the BTC solution. An asterisk on the XRD spectrum of ACT solvent indicates the Cu2OH(BTC)(H2O)n•2nH2O phase.47

Conclusions Here, we demonstrate the use of a conductive metal oxide thin film as an efficient and effective method of depositing MOFs onto various substrates, exploiting the ability of HDS to convert to MOFs. As a proof of concept, we have shown that Cu-BTC can be rapidly grown onto a


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conductive AZO layer at room temperature, avoiding tedious methods such as layer-by-layer deposition or the need for elevated temperatures and long reaction times. Importantly, the use of an AZO layer coupled with stabilizing polar aprotic solvents such as ACT, ACN, DMSO, or DMF ensure that Cu-BTC growth occurs only on the AZO surface, without competing Cu-BTC nucleation and growth steps occurring away from the substrate. Subsequently, this approach is both versatile and cost-effective; it is anticipated that this method could be extended to any substrate that is amenable to metal oxide deposition, and allows chemical reactants to be recycled because all reactions occur only on the substrate. Additionally, we have demonstrated that the Cu-BTC morphology can be optimized through careful choice of the Cu salt, solvent system, and pH, which are critical considerations for optimizing MOF structure for specific applications. We find that bulky Cu salt anions, such as acetate and sulfate, do not promote the growth of Cu-BTC, whereas smaller anions including chloride, bromide, and nitrate induce growth. Further, the morphology and packing density of the Cu-BTC changes as a function of anion identity; for example, poorly packed, micron-sized Cu-BTC structures are obtained when the chloride salt is used, whereas smaller, densely packed crystals form if the nitrate salt is used. Modest changes to the Cu-BTC size can be obtained by adding acids or bases to the BTC growth solution; larger crystals were obtained with NaOH additions, whereas the addition of HCl, which was found to inhibit Cu-BTC growth, yielded smaller structures. Finally, the use of more acidic polar aprotic solvents including ACN and ACT produced mixed morphologies observable by both XRD and SEM analysis, whereas DMF and DMSO produced only Cu-BTC crystals. In all cases, the presence of water was required to facilitate growth. Taken together, these experiments would provide powerful design rules for rapid, selective, and controlled Cu-BTC film growth on a variety of substrates, a critical step towards device integration. The ability to integrate MOF


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films onto various device platforms such as optical fibers or surface acoustic wave sensors are under way.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS publications website. It includes photographs of the Cu-BTC MOF films on the substrates, additional SEM images, and absorption spectra. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORICD Ki-Joong Kim: 0000-0001-9374-8103 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources SEC is grateful for financial support through the Department of Energy’s Mickey Leland Energy Fellowship program. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This technical effort was performed in support of the National Energy Technology Laboratory's ongoing research under the RES contract DE-FE0004000. This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with AECOM. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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For Table of Contents Use Only Rapid, Selective, Ambient Growth and Optimization of Copper Benzene-1,3,5Tricarboxylate (Cu-BTC) Metal-Organic Framework Thin Films on a Conductive Metal Oxide Scott E. Crawford, Ki-Joong Kim,* Yang Yu, and Paul R. Ohodnicki

Synopsis: Ambient, rapid, selective growth of copper benzene-1,3,5-tricarboxylate (Cu-BTC) metalorganic framework thin films on an aluminum zinc oxide is demonstrated. Control over crystal size and packing density is illustrated by altering synthetic parameters including copper salt identity, solvent conditions, and acid/base addition. The optimized, selective growth of MOF is a critical step towards developing devices for sensing gases.


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Rapid, Selective, Ambient Growth and Optimization of Copper

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