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  • Micromechanical Behavior of Polycrystalline Metal−Organic Framework Thin Films Synthesized by Electrochemical Reaction Imogen Buchan, Matthew R. Ryder, and Jin-Chong Tan*

    Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, United Kingdom

    *S Supporting Information

    ABSTRACT: We have studied the mechanical properties of an archetypical metal−organic framework (MOF) polycrystalline thin-film material, termed HKUST-1 or Cu3(BTC)2, which was synthesized by means of electrochemistry. We demonstrate that the average crystal size and surface coverage of electrochemically grown thin films, with associated coating thickness and surface roughness, can be controlled by adjusting not only the reaction time but also the anodic substrate surface characteristics. The polycrystalline films were characterized via scanning electron microscopy, optical three-dimensional profilometry, atomic force microscopy, and X-ray diffraction. Using an instrumented nanoindenter, we performed fine-scale nanoscratch experiments under two distinct test modes: (i) ramp-load and (ii) pass-and- return (cyclic wear), to establish the underpinning failure mechanisms of MOF coatings with varied average thicknesses (∼ 2−10 μm). Our results reveal that the ramp-load approach is ideal to pinpoint the critical force required to debond films from the substrate, and the pass-and-return method has the propensity to crush polycrystals into a compacted layer on top of the substrate, but cause no film debonding even at a high number of cycles. Notably the film-to-substrate adhesion strength of electrochemical coatings could be enhanced with increasing HKUST-1 film thickness (∼μm), while the attachment of polycrystals is weakened when grown on smoother substrates.

    1. INTRODUCTION Metal−organic framework (MOF)1 materials are three-dimen- sional (3D) open-framework structures2 constructed from the self-assembly of metal ions interconnected by organic linkages. In particular, the high porosity of MOFs combined with their tunable physicochemical properties has brought this new class of multifunctional materials, which bridges conventional micro- and mesoporous materials, to the forefront of materials science research.3,4 The potential opportunities opened up by the use of MOF-based materials are vast, spanning the disciplines of environmental, biomedical, energy and electronic engineering.5

    For example, their high porosity makes them a very good candidate for carbon capture and storage applications,6 as well as for targeted drug delivery,7 in which good structural stability is needed for controlled release of certain molecular substances. MOFs have also proved themselves as potential catalysts for important reactions8 and may function as active materials in chemical sensors,9 with their selectivity and sensitivity allowing them to detect and monitor specific substances or external stimuli. Multiple challenges need to be overcome before MOF

    materials can see commercial scale applications.3 The mechanical properties of MOFs, particularly when taking the form of thin films,10,11 are incredibly important in making potential MOF sensing devices a reality.12−14 This topic area, however, remains largely unexplored as research to date has

    focused elsewhere and thin films are a more recent develop- ment in the expanding field of MOF materials.3,12,13 Engineer- ing applications will often incur a combination of mechanical and thermal stresses during service, for example, tension- compression loading, bending, shear by torsion, cyclic loading and fatigue, many of which remain poorly understood to date.14

    On this basis, the desired mechanical requirements for an ideal MOF thin film include a damage-tolerant coating exhibiting strong interfacial cohesion properties, which could endure surface scratch and catastrophic cracking,10 abrasive wear,15 and impact delamination, among others. Electrochemical synthesis16 has been employed to generate

    the MOF thin-film coatings used in this study. Other reported methodologies for producing MOF thin films encompass layer- by-layer epitaxial growth,17 spray deposition,18 inkjet printing19

    to enable patterned deposition, and application of direct methods, e.g., dip coating in mother solution and in situ crystallization, and through post-assembly of preformed nanocrystals.12 The electrochemical method involves continu- ous introduction of metal ions through anodic dissolution, which then react with dissolved organic linker molecules in the presence of a conducting salt. This method is attractive not

    Received: January 31, 2015 Revised: March 8, 2015 Published: March 11, 2015

    Article

    pubs.acs.org/crystal

    © 2015 American Chemical Society 1991 DOI: 10.1021/acs.cgd.5b00140 Cryst. Growth Des. 2015, 15, 1991−1999

  • only because it has short reaction times, but also it operates under mild processing conditions10,20 and allows for morphol- ogy tuning,21 including generation of biphasic films.22 Addi- tionally, the synthesis route is thought to be dominated by homogeneous nucleation and growth, allowing continuous nucleation at all temperatures.13 While its limitation lies mainly in linker solubility, this can be improved by elevating temperatures so as to improve material yield.20

    This study focuses on thin-film coatings (defined as a few micrometers thickness) of a promising copper-based MOF material, known as HKUST-1 or Cu3(BTC)2 [BTC = 1,3,5- benzenetriboxylate], which consists of dimeric copper paddle wheels linked by 1,3,5-benzenetriboxylates. The nanoporous framework of HKUST-1 adopts a cubic crystal structure, featuring a nominal surface area of ∼2000 m2 g−1.23 It exhibits good stability against moisture, coupled with excellent thermal stability and straightforward synthesis appropriate for industrial scale up.24 It has been demonstrated that HKUST-1 has significant potential for a wide range of device-oriented applications in the field of chemical sensing and micro- electronics,25 where MOF thin-film structures and coatings are extremely relevant.26 Herein, we have fabricated HKUST-1 coatings on copper substrates via electrochemical synthesis, with which we have studied the influence of substrate condition and reaction time on the growth, coverage, and roughness of the resultant films, ultimately to build an understanding of the effects of synthetic conditions on the mechanical properties of HKUST-1 films. By means of systematic nanoscratch experi- ments supported by detailed microstructural characterization studies, we have established the behavior underpinning substrate-to-film adhesion and its failure mechanisms, grain size, film thickness, and interrelations between these poly- crystalline material parameters.

    2. EXPERIMENTAL METHODOLOGY 2.1. Electrochemical Synthesis of MOF Thin Films. The

    HKUST-1 films were grown on pure copper substrates, each measuring 30 × 10 mm2. Before electrochemical reactions, all substrates were ground through increasing SiC grit levels (240− 4000 max), using a metallographic grinder and polisher. The front face was prepared to the desired level for the given experiment, and the back face was prepared to a grit of 240 to remove any contamination on the copper surface. Specified substrates were then polished to 1, 3, or 6 μm using diamond suspensions on polishing cloths and appropriate lubricant. Polished substrates were sonicated for 10 min, submersed in acetone to remove contamination from the polishing lubricant, and eventually washed in ethanol prior to electrochemical synthesis. The electrochemical reaction was performed using an in-house

    designed electrochemical apparatus based on a recent study,10 as depicted in the Supporting Information (Figure S1). To ensure repeatability, 100 mL of fresh solution was prepared immediately prior to each reaction, comprising 56 mL of ethanol, 44 mL of deionized water, 1 g of 1,3,5-benzenetricarboxylic acid (ligand), and 2 g of tributylmethylammonium methyl sulfate (MTBS conduction salt), which were stirred until a clear solution was obtained. The solution was then heated to ∼55 °C, the electrodes were immersed in the solution, and a voltage of 2.5 V was applied. The HKUST-1 films grew on the inner face of the anode throughout the chosen reaction time (Table S1, Supporting Information). When reaction was completed, the anode with resultant coating was removed and washed with ethanol to eliminate excess ligands and Cu2+ ions. 2.2. Micromechanical Characterization. Nanoscratch experi-

    ments27 were performed using an MTS Nanoindenter XP system equipped with a Berkovich (three-sided pyramid) diamond tip. Two distinct nanoscratch modes were used: the ramp-load test and the

    pass-and-return test (i.e., cyclic wear). In each scratch test, three phases occur sequentially: (i) a very small load is applied to the surface in order to track and map out the original morphology of the sample surface; (ii) during the scratch phase, the same path is followed, but the specified normal load is applied; (iii) a very small load is applied once again in order to track and measure residual surface deformation along the scratch path after tip unloading (elastic recovery). In the results to follow, these three phases are represented by blue, green, and orange curves, respectively, for which the area between the green and orange curves corresponds to the extent of elastic recovery; the area encompassed between the orange and blue denotes (permanent) plastic deformation and/or fracture. We found that consistently higher surface penetration depths were recorded from scratch tests oriented at 180° compared to those aligned at 0° (Table S3, Supporting Information). This is thought to be caused by the pointed end of the Berkovich tip (Figure S2, Su