Micromechanical Behavior of Polycrystalline Metal−OrganicFramework 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 metal−organic 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 (∼ 2−10μm). 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 (∼μm), while the attachment of polycrystals is weakenedwhen grown on smoother substrates.

1. INTRODUCTIONMetal−organic 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.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 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

Article

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© 2015 American Chemical Society 1991 DOI: 10.1021/acs.cgd.5b00140Cryst. Growth Des. 2015, 15, 1991−1999

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 g−1.23 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 (240−4000 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 μm 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 therefore moresuccessfully detach and/or crush the crystals lying in its path. Giventhat a greater consistency was observed in the measurements obtainedfrom the 180° scratches, the ramp-load test results discussed herein areconcerned mainly with such orientation.

2.3. Surface Characterization of Polycrystalline Films andCoatings. We performed X-ray diffraction (XRD) on the thin-filmsamples produced to confirm that they were indeed thin-film coatingsof HKUST-1. The XRD patterns (Figure S3, Supporting Information)of the coatings were measured using the Rigaku Miniflexdiffractometer, for 2θ up to 30°. The diffraction data are consistentwith published crystallographic structure (CIF code: FIQCEN),23

confirming all coatings are HKUST-1; we observed that thinner filmsexhibit relatively lower intensities than their thicker counterparts, asexpected. The microstructures of the polycrystalline coatings obtainedwith different synthetic conditions were studied using scanningelectron microscopy (SEM: Carl Zeiss Evo LS15) to determinegrain size and distribution, film thickness, and nanoscratchdeformation. An optical noncontact 3D profilometer (Alicona InfiniteFocus) was used to characterize surface roughness, for which the 10×lens was used to allow the optimal combination of accuracy andavailable examination area. The profilometer was used to furtheridentify any sign of crystal detachment (removal) or deterioration afternanoscratch or wear experiments. The surface topography of thin filmswas also characterized by means of atomic force microscopy (AFM:Veeco Dimension 3100).

3. RESULTS AND DISCUSSION3.1. Effects of Electrochemical Reaction Parameters

on Resulting Thin-Film Coatings. SEM micrographspresented in Figures 1 and 2 show the impact of reactiontime and substrate surface conditions on the HKUST-1coatings produced. It is evident from Figure 1 that prolongingreaction time increases the overall size of the individual crystals.The film thicknesses, as a first estimate, were taken to be of thesame order as the average diameter of the crystals characterizedby SEM (Table S2, Supporting Information), assuming thatonly a single layer of crystals developed on the substrate surfacewith minimal piling up. Adopting this simple approach, theestimates for mean film thicknesses are given in Table 1. Thedata show film thickness increasing with reaction time asexpected; in the subsequent section, their accuracy will becompared against thickness values derived from an alternativeapproach utilizing nanoscratch experiments.Figure 2a shows that low levels of grinding may encourage

crystals to grow along the direction of the scratches generatedon the substrate surface, thereby leading to formation of a lessuniform polycrystalline film. Finer polishing of substrates couldeliminate such directional growth, producing more evenly

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distributed crystals, two examples are depicted in Figure 2c,d.However, it was observed that extensive (fine) polishing ofsubstrate could also encourage crystals to pile up on top of oneanother, suggesting crystal nucleation is stimulated in regionswhere (primary) crystals already exist, thus promotingsecondary formation.Non-contact 3D optical profilometry was used to determine

the average roughness (Ra) of the HKUST-1 thin-film coatings.Figure 3a shows that increasing the reaction time causes anincrease in surface roughness, approximately to a linearrelationship for conditions we studied. It is anticipated thatthe increasing film thickness (Table 1) contributes to thisobservation, as a larger number of crystals with increasingdiameter are likely to produce a rougher overall surfacecompared with fewer, smaller crystals. It is, however, noted thatfilms that do not completely cover the substrate may record anartificially high roughness due to height measurements of zerowhere crystals are absent on the surface. We furtherdemonstrate that coating surface roughness can be modifiedby changing initial substrate condition (Figure 3b), whereincreased levels of substrate polishing tend to generate a greaterfilm roughness. This result can be associated with the crystalpiling phenomenon depicted in micrographs of polishedsamples (see Figure 2).Coating B was chosen to investigate the efficacy of using

atomic force microscopy (AFM) to characterize the fine-scalesurface topography of electrochemically grown HKUST-1coatings. Figure 4 shows the surface roughness combinedwith micron-sized steps in changing surface heights betweenadjacent polycrystals. It is apparent that there is in fact a largevariation in the size of the individual crystals, which is a majorcontributor toward high surface roughness (Figure 3). Many ofthe crystals, especially the larger ones (which the AFMcantilever struggles to measure), exhibit inclined crystal facetscorresponding to the {111}-oriented facets prevalent inHKUST-1. Moreover, it is noted that relatively large crystalsappear to be surrounded by those of decreasing size down tobare areas of substrate. This finding supports the notion that acrystal piling effect is present in all samples, and it will befurther accentuated on thick denser coatings.

3.2. Nanoscratch Ramp-Load Test Behavior. (a)Coating A. Coating A represents the thinnest of theHKUST-1 polycrystalline films to be investigated here,synthesized using a relatively short reaction time of 1.5 min.In the 10 mN ramp-load test shown in Figure 5a, it can be seenthat the load−displacement data (green curve), representingthe indenter tip penetration into the coating during scratch test,transitions from one which is highly irregular, to a muchsmoother curve. This transition occurs around halfway throughthe test at a scratch distance of ∼100 μm, which corresponds toa normal scratch force of ∼5 mN. In fact, midway cross-

Figure 1. SEM micrographs of HKUST-1 thin-film coatings grownwith increasing reaction times: (a) coating A, 1.5 min; (b) coating B, 3min; (c) coating C, 10 min. Panels on the right show highermagnification images of the corresponding polycrystalline coatings. Allsamples were synthesized on pure copper substrates prepared to 2000grit surface finish.

Figure 2. SEM micrographs of thin-film coatings synthesized oncopper substrates with differing surface conditions: (a) 600 grit, (b)4000 grit, (c) 4000 grit and 6 μm polish with diamond suspension, (d)4000 grit and 1 μm polish with diamond suspension. All filmssynthesized using a reaction time of 3 min.

Table 1. Thickness of Thin-Film Coatings A, B, C, and DEstimated from SEM Micrographsa

thin filmcoating

thickness, t(μm)

substrate surface preparationmethod (grinding/polishing)

reactiontime (min)

A 2.33 ± 1.00 2000 grit 1.5B 3.70 ± 1.64 2000 grit 3C 9.87 ± 2.89 2000 grit 10D 9.71 ± 3.13 4000 grit followed by 1 μm polish

(diamond suspension)10

aThe raw data to derive mean values and standard deviations arepresented in the Supplementary Information (Table S2).

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sectional profiles presented in Figure 5b reveal that differentscratch tests have registered different penetration depths upontip unloading and elastic recovery. Micrographs of the residualscratches in Figure 5c,d show incomplete coverage of thepolycrystalline film, explaining the variation in force needed toremove the films and disparities of residual depths becausecrystals were only present sporadically along the line of thescratch test.(b) Coating B. Coating B represents a medium-thickness

film obtained using a reaction time of 3 min. Figure 6 showsthat the transition of the (green) load−displacement curve(from “rough” to “smooth” segments) occurs between 7 and 14mN in this sample. We may attribute this broader transitionrange to the increasing variability in film-to-substrate adhesionproperties of relatively thicker films, which are by naturerougher coatings. The cross-sectional profiles for the 15 mNramp test (Figure 6b) appear to be deeper and more consistentin shape than those of 10 mN (Figure S6), suggesting thesubstrate underneath might have been exposed by a force of∼7.5 mN. Nevertheless, when the scratches were examined

under the SEM (Figure 6c), it became clear that the films havenot been removed at this suspected transition point, nor at thehigher force of 15 mN. Intriguingly, in these thicker films, wefound that HKUST-1 crystals are crushed until only a thin layerof material exists on the surface, with a series of transversecracks propagating through it (Figure 6c).It can, however, be seen in Figure 7 that by means of the

substantially higher load of a 100 mN ramp test, the coppersubstrate can be reached, elucidating that this compressed layerof HKUST-1 crystals can in fact be completely removed byimposing greater forces. Notably, there are distinctive character-istics, present in the load−displacement graphs of films thathave been removed, to expose the bare substrate, which are notpresent in the data of those films which have been crushed toform a compacted layer. This finding enables us to determinethe nature of the film failure directly, without the need foradditional SEM examination. More precisely, we note in thevast majority of test cases (e.g., Figure 7a) which have led to anexposed substrate that the first 20% of the scratch distanceproduces a steeper surface penetration curve (green), whichreduces in gradient; for the remaining 80% of the scratchdistance, a distinctively linear displacement curve with aconstant gradient is obtained. Moreover, the elastic recoverycurve (orange) in the sample is approximately horizontal inthese cases (such that the area established between the greenand orange curves rises at a constant rate with the load ramp),thereby indicative of a force being applied purely to the coppersubstrate.

(c) Coating C. Specimen C represents a thick coating (t ≈ 10μm), obtained with a reaction time of 10 min. Figure 8a showsthat there is a substantial degree of elastic recovery beingdetected after indenter unloading (signified by a large areabound by the green and orange curves), suggesting thelikelihood of coating detachment. However, SEM examinations(Figure 8c) reveal that although all large crystals have indeedbeen cleared along the scratch path, this has led to a compactedlayer of crystals rather than complete exposure of theunderlying substrate. This result follows logically from thesame discovery relating to coating B, as it may be predicted thatsuch a failure mechanism (i.e., polycrystalline crushing) is morepronounced in thicker electrochemical films.Figure 8b shows the cross-sectional profiles for the 80 mN

ramp test, where a variation in penetration depths can beascribed to its greater surface roughness combined with lack ofuniformity in thicker films. Interestingly, the highest ramp testattempted for this sample was at 150 mN, at which it was foundthat the crushed layer had still not been completely removed toexpose the copper substrate. Our results reveal that thickerHKUST-1 films fabricated via an electrochemical route canexhibit a stronger mechanical adhesion to the surface,combined with a higher tendency to crush into a compactedcrystal layer than to fail altogether (e.g., coating delamination)to reveal the bare substrate surface.

(d) Coating D. Coating D (t ≈ 10 μm) was studied toinvestigate the effects of substrate polishing on coating-to-substrate adhesion property and failure mode. It is striking tosee that the 60 mN ramp test results for coating D (Figure 9)show considerably greater crushing damage accompanied by aconsiderably lower elastic recovery than the 80 mN resultsfound for coating C (Figure 8), thereby polycrystals on coatingD appear to be more susceptible to this failure mechanism. Infact, tests performed at 80 mN on coating D (Figure S8 inSupporting Information) confirmed that the crushed layer of

Figure 3. (a) Correlation between the reaction time used inelectrochemical synthesis and average surface roughness of a seriesof samples varying from reaction times of 1.5−10 min. Substratesurfaces were prepared to 2000 grit in all cases. Thin film coatings A,B, and C are highlighted. The linear regression line can be expressed asRa (nm) = 83.8476 × reaction time (min) with an R2-value of 0.98201.(b) Correlation between the substrate surface condition and averageroughness of a series of samples varying from 600 grit to 4000 grit and1 μm polish using diamond suspensions. In all cases a reaction time of3 min was used. Each data point corresponds to an average of five testsspread evenly across the film covering an area of 1 mm × 4.4 μm foreach. Note: red curves serve as guides for the eye.

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HKUST-1 crystals will remain attached to the substrate athigher loads. Particularly, the corresponding cross-sectionalprofiles of coating D (Figure 9b) are more consistent thanthose of coating C, revealing that crystals have beenpermanently deformed more readily from the wear track.This finding suggests that excessive (fine) polishing of thesubstrate prior to electrochemical synthesis might decrease thefilm-to-substrate adhesion strength, although it makes almostno visual difference with regard to the microstructural

appearance of thicker coatings under the SEM (c.f. comparisonpresented in Figure S4 in Supporting Informatiom).

3.3. Determination of Film Thickness from Nano-scratch Experiments. Utilizing the load−displacement datarecorded by the nanoindenter as the nanoscratch test takesplace, an estimate can be derived for the thickness values of theHKUST-1 films. This was achieved by averaging surfacepenetration depths for all nanoscratch tests in which thesubstrate was reached (Table S3). Nevertheless, in cases wherethis was not achieved, values were used from the nanoscratch

Figure 4. AFM topography of coating B determined at (a) 20 × 20 μm2 and (b) 10 × 10 μm2, showing inclined crystal facets and surface heightinformation. Coating B was synthesized on a copper substrate prepared to 2000 grit with a reaction time of 3 min.

Figure 5. (a) Load vs displacement graph of a 10 mN ramp load nanoscratch test on thin-film coating A over a defined scratch distance of 200 μm,with the indenter tip oriented at 180°. Blue, green, and orange curves correspond to the initial surface scan, linear ramp scratch phase, and residualprofile scan after tip unloading, respectively. (b) Midway cross-sectional profiles (corresponding to a scratch load 5 mN) of five independentnanoscratch measurements performed on coating A (upon unloading), mapped transversely to the scratch path using a cross profile distance of 30μm. (c) SEM micrograph of a 10 mN ramp load nanoscratch test on coating A, and (d) the corresponding image obtained by 3D opticalprofilometer. Coating A was synthesized on a copper substrate prepared to a surface finish of 2000 grit, using a reaction time of 1.5 min.

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tests which caused significant crushing of the films instead,hence producing an underestimation of the film thickness forsuch cases, i.e., coatings C and D. Figure 10 shows the results ofthis analysis, when compared to those estimated from SEMmicrographs presented in Table 1. It can be seen that thenominal values derived from the two approaches arecomparable, signifying they possess a reasonable level ofaccuracy. However, the standard deviation is higher for thickercoatings, for reasons elucidated above.3.4. Nanoscratch Pass-and-Return Wear Test Behav-

ior. Pass-and-return wear experiments were performed oncoating C to study how application of a repetitive tangential

force (cyclic stress) affects film-to-substrate adhesion and toprobe associated failure mode. Coating C was chosen because itis the thickest coating (t ≈ 10 μm) and also represents a samplethat has not been debonded from the substrate as the result oframp-load tests. The pass-and-return method applies a constantnormal force in both the 0° and 180° orientations of the scratchpath. In this study, 50 repetitions were undertaken at 10, 30,and 50 mN as shown in Figure 11. It can be seen that, asexpected, higher forces require fewer passes to reach a steadystate in surface penetration depth. It is evident from the SEMmicrographs in Figure 12 that, for all of the forces tested here,the steady state is one in which the indenter tip is passing over

Figure 6. (a) Load vs displacement graph of a 15 mN ramp load nanoscratch test on coating B over a scratch distance of 200 μm. (b) Midway cross-profiles at 7.5 mN of five separate tests. SEM micrographs of (c) a 15 mN ramp load nanoscratch test on coating B, and (d) its corresponding 3Doptical profilometer image. Coating B was synthesized on a copper substrate prepared to 2000 grit using a reaction time of 3 min.

Figure 7. (a) Load vs displacement graph of a 100 mN ramp load nanoscratch test on coating B over a scratch distance of 100 μm. (b) Midwaycross-profiles corresponding to a normal load of 50 mN. (c) SEM micrograph showing the fully exposed copper substrate, and (d) its opticalprofilometer image.

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a crushed layer of crystals which remains attached to thesubstrate. This is indicative of a certain level of elastic recoveryoriginating from the crushed HKUST-1 films; the slowcompression of this compacted MOF layer is responsible forsuppressing rapid increase in magnitude of the displacementinto the substrate as the repetitions progress. On the contrary,if the film had been removed, one would expect every pass toreach the same maximum displacement into the surface.Indeed, the pass-and-return test is less effective at detaching thecoating from the substrate than the ramp-load scratch test, such

that the crushed layer of MOF material may protect thesubstrate from further abrasion damage during cyclic loading.

4. CONCLUSIONSThin films and coatings of MOFs have a large number ofpotential applications spanning several fields of technology, yetrelatively little is understood about their detailed mechanicalbehavior in such configurations. In this work, we have focusedon HKUST-1, particularly its thin film structures generated byelectrochemical reactions, because this nanoporous framework

Figure 8. (a) Load vs displacement graph of a 80 mN ramp load nanoscratch tests on coating C over a scratch distance of 200 μm. (b) Midwaycross-profiles corresponding to 40 mN. (c) SEM micrograph of a 80 mN ramp load nanoscratch tests on thin film coating C, and (d) thecorresponding optical profilometer image. Coating C was synthesized on a copper substrate prepared to 2000 grit with a reaction time of 10 min.

Figure 9. (a) Load and displacement data from a 60 mN nanoscratch ramp-load test on coating D over a scratch distance of 200 μm. (b) Midwaycross-profiles at 30 mN over a profile distance of 30 μm. (c) SEM micrograph of a 60 mN ramp load nanoscratch test on coating D, and (d) itscorresponding 3D optical profilometer image. Coating D was synthesized with a reaction time of 10 min on a copper substrate prepared to 4000 gritfollowed by 1 μm polish using a diamond suspension.

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represents one of the most promising and widely studied MOFmaterials today.• There is scope to control the film thickness, polycrystalline

coverage, and overall surface roughness of HKUST-1 coatingsvia electrochemistry. While increasing the reaction timeincreases the thickness and coverage of the thin-film coating,this also increases surface roughness through both an increasein average crystal size together with secondary crystal growth.• Metal substrate surface condition prior to electrochemical

synthesis could affect the properties of the film produced.Specifically, increased levels of grinding increase the quality ofthe film, but grinding alone encourages crystals to grow onlyalong the lines of the scratches left on the substrate, which canlead to a sparser and less uniform surface coverage. Polishingcauses the crystals to pile up, which can increase surfaceroughness. The qualitative effect of polishing on thickercoatings (t ∼10 μm) is negligible as the crystal growth evensout over time; however, we found that the film-to-surfaceadhesion strength of films synthesized on polished substrates isrelatively weaker.• Nanoscratch experiments have established two dominant

failure mechanisms for the thin-film coatings, with ramp-loadtests identifying the force required to remove coatings ofdifferent thicknesses from the substrate and pass-and-return

(wear) tests exhibiting a tendency to crush the crystals into acompacted layer on top of the substrate. The results showincreasing film-to-substrate adhesion with increasing filmthickness, and 180° scratch tests were found to be moreconsistent on these rough materials and more successful atcompletely removing the films.• Accurate measurement of MOF film thickness is

challenging especially for rough polycrystalline coatings. Thisstudy demonstrates that nanoscratch testing provides a possiblemeans to approximating the coating thickness on the basis ofthe surface penetration information acquired from the ramp-load test.• We have demonstrated the efficacy of nanoscratch

techniques to systematically characterize the mechanicalproperties of MOF thin films, in particular, large straindeformation under tangential forces (scratch), crystal-to-substrate attachment strengths, and the underpinning failuremechanisms. It is envisaged that the above methodologiescould be extended to probe the mechanical properties of a widerange of MOF thin films and polycrystalline coatings useful toemerging device applications.

Figure 10. Thicknesses of thin film coatings estimated from SEMimages and those determined based on nanoscratch ramp-load testing(Table S3). Coatings A, B, and C were synthesized on a coppersubstrate prepared to 2000 grit using reaction times of 1.5, 3, and 10min, respectively. Coating D was synthesized with a reaction time of10 min, on a copper substrate prepared to 4000 grit and 1 μm polishusing a diamond suspension.

Figure 11. Displacement vs scratch distance plots of (a) 10 mN, (b) 30 mN, and (c) 50 mN obtained from pass-and-return nanoscratch wear testsperformed on coating C, over a scratch distance of 100 μm each way. Each test features 50 cycles. Coating C was synthesized on a copper substrateprepared to 2000 grit with a reaction time of 10 min.

Figure 12. SEM micrographs of a 50 mN pass-and-return nanoscratchwear test on coating C after subjected to 50 wear cycles, each of whichtraversed a distance of 100 μm. The scratch path is enclosed in themarked region.

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■ ASSOCIATED CONTENT*S Supporting InformationAdditional figures, micrographs and supporting data. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: jin-chong.tan@eng.ox.ac.uk.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge the Maurice Lubbock Memorial Fund(DF8100-2013b/3, Balliol College, University of Oxford) andthe Royal Society Research Grants (RG140296) for researchfunding. We thank Mr. Ben Van de Voorde at K.U. Leuven forhelpful advice concerned with construction of the electro-chemical setup and its processing parameters. We thank Prof.Steve Roberts and Dr. David Armstrong for kindly granting usaccess to the nanoindentation testing facilities in the Depart-ment of Materials at the University of Oxford. We are gratefulto the ISIS Facility, Rutherford Appleton Laboratory, especiallyDr. Marek Jura and Dr. Gavin Stenning at R53 MaterialsCharacterisation Laboratory, and Dr. James Taylor at R79Hydrogen and Catalysis Laboratory for allowing us access toXRD and spectrometers. We would also like to acknowledgethe provision of advanced materials characterization facilities bythe Research Complex at Harwell (RCaH), RutherfordAppleton Laboratory, Oxfordshire.

■ ABBREVIATIONSMOF, metal−organic framework; BTC, benzene-1,3,5-tricar-boxylate; MTBS, tributylmethylammonium methyl sulfate

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Crystal Growth & Design Article

DOI: 10.1021/acs.cgd.5b00140Cryst. Growth Des. 2015, 15, 1991−1999

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