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Micromechanical Behavior of Polycrystalline MetalOrganicFramework Thin Films Synthesized by Electrochemical ReactionImogen 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 anarchetypical metalorganic framework (MOF) polycrystallinethin-film material, termed HKUST-1 or Cu3(BTC)2, which wassynthesized by means of electrochemistry. We demonstrate thatthe average crystal size and surface coverage of electrochemicallygrown thin films, with associated coating thickness and surfaceroughness, can be controlled by adjusting not only the reactiontime but also the anodic substrate surface characteristics. Thepolycrystalline films were characterized via scanning electronmicroscopy, optical three-dimensional profilometry, atomic forcemicroscopy, and X-ray diffraction. Using an instrumentednanoindenter, we performed fine-scale nanoscratch experimentsunder 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 ( 210m). Our results reveal that the ramp-load approach is ideal to pinpoint the critical force required to debond films from thesubstrate, 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 electrochemicalcoatings could be enhanced with increasing HKUST-1 film thickness (m), while the attachment of polycrystals is weakenedwhen grown on smoother substrates.
1. INTRODUCTIONMetalorganic framework (MOF)1 materials are three-dimen-sional (3D) open-framework structures2 constructed from theself-assembly of metal ions interconnected by organic linkages.In particular, the high porosity of MOFs combined with theirtunable physicochemical properties has brought this new classof multifunctional materials, which bridges conventional micro-and mesoporous materials, to the forefront of materials scienceresearch.3,4 The potential opportunities opened up by the useof MOF-based materials are vast, spanning the disciplines ofenvironmental, biomedical, energy and electronic engineering.5
For example, their high porosity makes them a very goodcandidate for carbon capture and storage applications,6 as wellas for targeted drug delivery,7 in which good structural stabilityis needed for controlled release of certain molecular substances.MOFs have also proved themselves as potential catalysts forimportant reactions8 and may function as active materials inchemical sensors,9 with their selectivity and sensitivity allowingthem to detect and monitor specific substances or externalstimuli.Multiple challenges need to be overcome before MOF
materials can see commercial scale applications.3 Themechanical properties of MOFs, particularly when taking theform of thin films,10,11 are incredibly important in makingpotential MOF sensing devices a reality.1214 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 mechanicaland thermal stresses during service, for example, tension-compression loading, bending, shear by torsion, cyclic loadingand fatigue, many of which remain poorly understood to date.14
On this basis, the desired mechanical requirements for an idealMOF thin film include a damage-tolerant coating exhibitingstrong interfacial cohesion properties, which could enduresurface scratch and catastrophic cracking,10 abrasive wear,15 andimpact delamination, among others.Electrochemical synthesis16 has been employed to generate
the MOF thin-film coatings used in this study. Other reportedmethodologies for producing MOF thin films encompass layer-by-layer epitaxial growth,17 spray deposition,18 inkjet printing19
to enable patterned deposition, and application of directmethods, e.g., dip coating in mother solution and in situcrystallization, and through post-assembly of preformednanocrystals.12 The electrochemical method involves continu-ous introduction of metal ions through anodic dissolution,which then react with dissolved organic linker molecules in thepresence of a conducting salt. This method is attractive not
Received: January 31, 2015Revised: March 8, 2015Published: March 11, 2015
2015 American Chemical Society 1991 DOI: 10.1021/acs.cgd.5b00140Cryst. Growth Des. 2015, 15, 19911999
only because it has short reaction times, but also it operatesunder 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 byhomogeneous nucleation and growth, allowing continuousnucleation at all temperatures.13 While its limitation lies mainlyin linker solubility, this can be improved by elevatingtemperatures so as to improve material yield.20
This study focuses on thin-film coatings (defined as a fewmicrometers thickness) of a promising copper-based MOFmaterial, known as HKUST-1 or Cu3(BTC)2 [BTC = 1,3,5-benzenetriboxylate], which consists of dimeric copper paddlewheels linked by 1,3,5-benzenetriboxylates. The nanoporousframework of HKUST-1 adopts a cubic crystal structure,featuring a nominal surface area of 2000 m2 g1.23 It exhibitsgood stability against moisture, coupled with excellent thermalstability and straightforward synthesis appropriate for industrialscale up.24 It has been demonstrated that HKUST-1 hassignificant potential for a wide range of device-orientedapplications in the field of chemical sensing and micro-electronics,25 where MOF thin-film structures and coatings areextremely relevant.26 Herein, we have fabricated HKUST-1coatings on copper substrates via electrochemical synthesis,with which we have studied the influence of substrate conditionand reaction time on the growth, coverage, and roughness ofthe resultant films, ultimately to build an understanding of theeffects of synthetic conditions on the mechanical properties ofHKUST-1 films. By means of systematic nanoscratch experi-ments supported by detailed microstructural characterizationstudies, we have established the behavior underpinningsubstrate-to-film adhesion and its failure mechanisms, grainsize, film thickness, and interrelations between these poly-crystalline material parameters.
2. EXPERIMENTAL METHODOLOGY2.1. Electrochemical Synthesis of MOF Thin Films. The
HKUST-1 films were grown on pure copper substrates, eachmeasuring 30 10 mm2. Before electrochemical reactions, allsubstrates were ground through increasing SiC grit levels (2404000 max), using a metallographic grinder and polisher. The front facewas prepared to the desired level for the given experiment, and theback face was prepared to a grit of 240 to remove any contaminationon the copper surface. Specified substrates were then polished to 1, 3,or 6 m using diamond suspensions on polishing cloths andappropriate lubricant. Polished substrates were sonicated for 10 min,submersed in acetone to remove contamination from the polishinglubricant, and eventually washed in ethanol prior to electrochemicalsynthesis.The electrochemical reaction was performed using an in-house
designed electrochemical apparatus based on a recent study,10 asdepicted in the Supporting Information (Figure S1). To ensurerepeatability, 100 mL of fresh solution was prepared immediately priorto each reaction, comprising 56 mL of ethanol, 44 mL of deionizedwater, 1 g of 1,3,5-benzenetricarboxylic acid (ligand), and 2 g oftributylmethylammonium methyl sulfate (MTBS conduction salt),which were stirred until a clear solution was obtained. The solutionwas then heated to 55 C, the electrodes were immersed in thesolution, and a voltage of 2.5 V was applied. The HKUST-1 films grewon 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 withethanol to eliminate excess ligands and Cu2+ ions.2.2. Micromechanical Characterization. Nanoscratch experi-
ments27 were performed using an MTS Nanoindenter XP systemequipped with a Berkovich (three-sided pyramid) diamond tip. Twodistinct nanoscratch modes were used: the ramp-load test and the
pass-and-return test (i.e., cyclic wear). In each scratch test, threephases occur sequentially: (i) a very small load is applied to the surfacein order to track and map out the original morphology of the samplesurface; (ii) during the scratch phase, the same path is followed, butthe specified normal load is applied; (iii) a very small load is appliedonce again in order to track and measure residual surface deformationalong the scratch path after tip unloading (elastic recovery). In theresults to follow, these three phases are represented by blue, green, andorange curves, respectively, for which the area between the green andorange curves corresponds to the extent of elastic recovery; the areaencompassed between the orange and blue denotes (permanent)plastic deformation and/or fracture. We found that consistently highersurface penetration depths were recorded from scratch tests orientedat 180 compared to those aligned at 0 (Table S3, SupportingInformation). This is thought to be caused by the pointed end of theBerkovich tip (Figure S2, Supporting Information), which leads in the0 scratch tests, less successfully debonding crystals from the surface,while the blunt end of the tip which leads in the 180 scratch tests hasa larger surface area with which to apply the force and the