Heterogeneous Catalysis “On Demand”: Mechanically Controlled

Letter Cite This: ACS Appl. Mater. Interfaces 2017, 9, 44264−44269

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Heterogeneous Catalysis “On Demand”: Mechanically Controlled Catalytic Activity of a Metal Surface Tomasz Mazur,† Slawomir Lach,† and Bartosz A. Grzybowski* IBS Center for Soft and Living Matter and the Department of Chemistry, Ulsan National Institute of Science and Technology, Ulsan, South Korea S Supporting Information *

ABSTRACT: A metal surface passivated with a tightly packed selfassembled monolayer (SAM) can be made catalytically active upon the metal’s mechanical deformation. This deformation renders the SAM sparser and exposes additional catalytic sites on the metal’s surface. If the deformation is elastic, return of the metal to the original shape “heals” the SAM and nearly extinguishes the catalytic activity. Kelvin probe force microscopy and theoretical considerations both indicate that the catalytic domains “opening up” in the deformed SAM are of nanoscopic dimensions. KEYWORDS: heterogeneous catalysis “on demand”, self-assembled monolayer, mechanical deformation, catalytic domains atalysts that can be turned “on” and “off” by external stimuli have become the focus of recent research and are often considered as control elements of futuristic chemical systems/networks, in which reaction sequences could be controlled by the sequences of the stimuli applied.1−3 To date, “switchable” homogeneous catalysts have been demonstrated based on photoresponsive moieties (azobenzenes,4−7 dihydrophenazines,8 and bicyclopentylidenes9,10) and macromolecular complexing agents (e.g., β-cyclodextrins11,12 and cavitands13) as well as various metal−organic scaffolds whose conformations and catalytic properties depend on the pH,14 intramolecular coordination,15 ligand binding,16 dimerization,17 and more (for an excellent recent review, see Blanco et al.18). In heterogeneous catalysis, our group demonstrated a system in which the catalytic activity of metal nanoparticles depends on their aggregation, which, in turn, is controlled by photoisomerizable surface ligands.19 A separate effort has been devoted to modulation of the catalytic activity by mechanical stimuli. In particular, the group of Sijbesma developed a range of polymers with covalently attached N-heterocyclic carbinebased catalytic moieties and coordinated to, for instance, silver(I)20 or ruthenium(II);21,22 the application of either a mechanical force or ultrasound caused scission of the coordination bonds, thus activating the catalysts toward various types of reactions (e.g., transesterification, ring-closing metathesis, or ring-opening metathesis polymerization). Perhaps not unexpectedly, similar mechanochemical phenomena have not been extended to materials such as metals that are much harder to deform and cannot be easily made catalytic/noncatalytic based on the application of mechanical forces alone. Yet, as we demonstrate here, catalytic metals covered with self-assembled monolayers (SAMs)23,24 can also serve as rudimentary switchable catalysts. Specifically, we describe a system in which elastic deformation of a metal surface creates

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additional surface area and effectively makes the SAM supported by this surface sparser (but not “cracked” at the mesoscopic level), allowing access of the reagents to the metal’s catalytic surface (Figure 1). Remarkably, when the load is removed and the metal returns to its original shape, the protective SAM “heals” and catalysis nearly ceases. These phenomena are reproducible in the Hookean regime and suggest a new way of controlling heterogeneous catalysis on deformable surfaces that can be stabilized with protective monolayers. The catalytic element in our experiments was a platinum (Pt) wire of 250 μm diameter (Sigma-Aldrich, catalog no. 267171). The wire was cut into ∼2-cm-long segments, which were then thoroughly cleaned by washing in a piranha solution [3:1 (v/v) 96% H2SO4/30% hydrogen peroxide (H2O2)] for at least 30 min, followed by mechanical polishing with an abrasive paper (increasing the paper’s gradation stepwise from P600 to P1200) and then with a polishing paste (Buehler; particle size 0.05 μm). Next, the wires were sonicated in water (H2O; 10 min), immersed in a diluted (30%) aqua regia solution [1:3 (v/v) HNO3/HCl] for 30 s, and finally sonicated in H2O and in ethanol for 10 min each (see Figure S1 for images of wires at different stages of polishing/cleaning). Thiol monolayers were prepared according to a previously reported procedure,25 by immersing the Pt rod in 16 mM ethanolic solutions (200 proof, Sigma-Aldrich, degassed for 30 min prior to use) of various thiols for 14 days. Subsequently, the wires were thoroughly rinsed and kept immersed in ethanol. Five thiols were tested: 1hexanethiol, 1-dodecanethiol, 1-octadecanethiol, thiophenol, and 1-naphthalenethiol. Of these, 1-naphthalenethiol was Received: October 11, 2017 Accepted: November 27, 2017 Published: November 27, 2017 44264

DOI: 10.1021/acsami.7b15253 ACS Appl. Mater. Interfaces 2017, 9, 44264−44269

Letter

ACS Applied Materials & Interfaces

Figure 1. Principle and realization of mechanically controlled heterogeneous catalysis. (a) A thin Pt rod covered with a tightly packed SAM of alkanethiolates is catalytically inactive toward H2O2 present in the surrounding medium. (b) When, however, the rod is slightly bent, the SAM becomes sparser and the Pt-catalyzed decomposition of H2O2 produces oxygen bubbles. The arrow between schemes (a) and (b) means that the activation/exposure of the metal surface is reversible, provided that deformation of the rod is small. (c) Elements of the device used for precise rod bending, including the Thorlabs MTS50-Z8 motorized stage, a set of custom-designed 3Dprinted elements, and a glass capillary. (d) Fully assembled setup. (e) SolidWorks design and (f) experimental top views of the bending device. The SAM-covered Pt wire is pressed (in the images, from right to left) against a vertically positioned glass capillary (visible as a small circle in the middle). (g) Experimental images of the Pt rod covered with the SAM of 1-naphthalenethiol, immersed in the 0.5% H2O2 solution (changed every 20 min, at every bending increment ΔL, to ensure that the concentration of H2O2 remained constant). In the initial state (left), the rod is unreactive. Bending of the rod (middle) exposes the catalytic surface and initiates a reaction, which manifests itself by the formation of oxygen bubbles. (right) When the compressive force is removed and the rod unbends, catalysis ceases.

Figure 2. Rates of the mechanically controlled decomposition of H2O2. (a) The semilogarithmic concentration versus time plot evidences first-order kinetics. The data are for the minimal bending of the wire at which evolution of the oxygen bubbles is observed. Error bars correspond to standard deviations from six independent experiments. The reaction rate constant determined from the slope of the best-fitting line is k = 1.23 × 10−4 s−1. (b) Approximately linear dependence of the relative rates of catalysis on the additional surface area created during bending. Surface areas were calculated by FEM (see the main text); the additional surface area at the deformation at which the first bubbles are observed corresponds to 100%. (c) Relative rates of the catalytic activity during the wire’s sequential bending and unbending. The value for the first bending is taken as 100%. For the first unbending, the small red marks indicate the rate values for the individual wires (with three wires being completely reversible and showing no catalytic activity). All error bars are standard deviations from six independent experiments.

SAM. Experiments described in the following are therefore for the system of a Pt wire covered with 1-naphthalenethiol and, unless stated otherwise, immersed in a 0.5% solution of H2O2. The key assumption of our work is that it should be possible to control the catalytic activity by bending the SAM-covered Pt wire, thus changing the surface area of the metal and “openingup” an additional catalytic surface (Figure 1a,b). When the bending is very pronounced and/or uncontrolled, however, the SAM is permanently damaged, and the Pt surface remains exposed and catalytic even when the macroscopic shape of the

chosen as the SAM because it prevented any detectable reactions on unbent wires (cf. below and Figure S2). Several Ptcatalyzed reactions were also investigated−decomposition of H2O2, decomposition of NaBH4 coupled with 4-nitrophenol reduction, and decomposition of formic acid [for details, cf. the Supporting Information (SI), section S4]. Of these, the simple Pt

decomposition of H2O2 on Pt, 2H 2O2 → 2H 2O + O2 (g), was chosen because it evolved easily detectable oxygen bubbles off of any portions of the wires that were not protected by the 44265

DOI: 10.1021/acsami.7b15253 ACS Appl. Mater. Interfaces 2017, 9, 44264−44269

Letter

ACS Applied Materials & Interfaces

noncatalytic, because some sporadic, small bubbles are observed on some, but not all, wire samples; see below). In order to quantify these effects, we studied reaction kinetics at the minimal bending of the wire, at which oxygen bubbles appear (i.e., at Lcrit). The amount of evolved oxygen was determined from the sizes of the bubbles, using the Laplace equation, pi = 2γ/R, where pi stands for the pressure inside the bubble, γ ∼ 7.28 × 10−2 N/m is the surface tension of the air/ H2O interface, and R is the bubble’s radius. The semilogarithmic plot in Figure 2a evidences a linear dependence between the logarithm of the concentration of the peroxide (obtained from the measured concentration of the evolved oxygen via reaction stoichiometry) and time; that is, the reaction is first-order in H2O2, as is indeed expected.26 Next, we confirmed that the catalytic activity is proportional to the surface area that is being exposed during bending. In these experiments, we first established the reaction rate at the displacement Lcrit (cf. above) and gave it a reference value of 100%. We then increased the degree of bending until the limit of the elastic regime. For each deformation, L, we used finite element modeling (FEM; see the SI, section S5) to calculate the newly created surface area A(L). When the reaction rate was plotted against A(L), the dependence was not only linear (as should be expected27) but close to the y = x line [i.e., with a slope close to unity extrapolating to (0, 0); Figure 2b], indicating that the entire newly created area is available for catalysis. With these preliminaries, we examined whether and when the catalytic activity can be reversibly turned “on” and “off”. The data in Figure 2c are for six different wires, each subject to three consecutive cycles of elastic bending/unbending. As seen, in their initial states, all wires were catalytically inactive. After the first bending to Lcrit ∼ 0.6 mm, the wires became catalytically active, with the value of the average rate marked in the plot as 100% reference. After the first unbending, catalysis nearly ceased, with the average rate dropping to 11%. In fact, three out of the six rods tested were fully reversible, meaning that they showed no catalysis at all; the nonzero rate is due to the fact that the formation and quality of the SAM varies between the Pt wire samples. The second bending of the wire restored the catalytic activity to roughly the previously observed levels. On the second unbending, however, catalysis could not be completely switched off and remained at ∼50%, signaling some irreversible deterioration of the SAM (and possibly the wire’s deformation becoming plastic rather than elastic). Finally, the third bending caused the activity to increase, but the activity difference between the “straight” and “bent” states decreased. In principle, the switching described above could be ascribed to the changes in the properties of the Pt alone and independent of the SAM. In fact, it is known that catalytic metals under strain can change the grain structure (or even the electronic structure28) and can become more active. To examine the influence of such effects, we performed a series of control experiments with bare Pt wires. We had to decrease the concentration of H2O2 because, at 0.5% (as in the experiments with SAM-covered samples), the large numbers of rapidly forming oxygen bubbles precluded meaningful quantification of the differences between straight and slightly bent wires. Such a quantification was possible when the concentration of H2O2 was lowered by 2 orders of magnitude; under these conditions,29 the rate constant for the peroxide decomposition

Figure 3. Control experiments. (a) Decomposition of H2O2 on straight versus bent bare Pt wires. The concentration of H2O2 was 0.005%, and bending was kept within the same regime as that for the SAM-covered samples (motion up to Lmax ∼ 0.6−0.8 mm). The semilogarithmic concentration versus time plot evidences first-order kinetics, with the reaction rate constants determined from the slope. Error bars are based on six independent experiments. (b) Decomposition of H2O2 on similarly bent (Lmax ∼ 0.6−0.8 mm) bare Pt versus SAM-protected wires. The concentration of H2O2 was 0.3% and was chosen to produce, on similar time scales, some bubbles on the SAM-protected wire while not producing excessive bubbles (obscuring quantification) on bare Pt samples. Error bars are based on three independent experiments.

wire is restored (see Figure S3); that is, it is possible to switch catalysis “on” but not back “off”. With this in mind, we constructed a mechanical system that controlled the wire’s bending precisely (Figure 1c−f). As key elements, this system comprised a 3D-printed (3Dison Rokit Pro printer; SolidWorks software) “wedge” attached to a motorized translational stage (Thorlabs MTS50-Z8) offering 0.05 μm precision of motion. When the stage moved, the wedge pressed and bent the wire against a vertically positioned glass capillary (diameter 1.5 mm); when the stage was retracted, the wire was unbending. The bending−unbending was reversible in the Hookean/elastic regime (up to Lmax ∼ 0.8−1.0 mm motion of the translational stage). Figure 1g illustrates control of the catalytic activity thus achieved; the unbent rod in the left portion of the figure is not evolving any oxygen bubbles, even if kept in the peroxide solution for tens of minutes. Then, the rod is gradually being bentby moving the translational stage at increments of ΔL = 100 μmand kept at each position for at least 20 min. Initially, no bubbles appear, but catalysis finally kicks off at Lcrit = 0.6− 0.8 mm. Retraction of the translational stage and unbending of the rod returns the system to the noncatalytic state (or nearly 44266

DOI: 10.1021/acsami.7b15253 ACS Appl. Mater. Interfaces 2017, 9, 44264−44269

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ACS Applied Materials & Interfaces

Figure 4. KPFM measurements of the surface potential of Au-on-PMMA (15 mm × 15 mm foil) microcontact-printed with 6-μm-wide lines of 1naphthalenethiol. The images in the left column are for the unbent surface; those in the middle column are for the surface bent by ∼80°; those in the right column are for the surface returned to its original, unbent shape. (a, c, and e) KPFM scans of the SAMs micropatternedas an array of parallel lineson Au/PMMA films. (b, d, and f) Corresponding histograms of the surface potential over regions marked with red boxes in parts a, c, and e. The KPFM measurements were performed on a Bruker Dimension Icon SPM microscope. PFQNE-AL probes were used: tip radius curvature, ≤12 nm; spring constant, 0.8 N/m; resonant frequency, 300 kHz. Experiments were performed under ambient conditions, with a relative humidity of ca. 40% and a temperature of 22 °C.

reaction was k = 4 × 10−4 s−1 for straight wires versus k = 7 × 10−4 s−1 for bent ones (Figure 3a). These rate values are quite similar and cannot account for the drastic none-to-full-catalysis change observed with SAM-protected Pt. We observe that the catalytic activity did not decrease when the bent bare Pt wires were unbent; that is, the bare metal surface cannot “heal” itself like the SAM-covered one. Another important comparison to make is between similarly bent bare Pt and SAM-protected wires. Because the applied strains are on the order of 1%, one would expect the net catalytic activity of the latter to be, at most, 1% of the bare Pt control. Indeed, measurements quantified in Figure 3b confirm that the activities differed by slightly more than 2 orders of magnitude with rate constants k = 3.79 × 10−3 s−1 for bare Pt and k = 3.62 × 10−5 s−1 for the SAM-protected samples. With these considerations in mind, one might envision three general scenarios for the deformation-driven “opening up” of the SAM to expose an additional catalytic surface: (1) uniform expansion of the monolayer, (2) formation of large (tens of nm), localized cracks, or (3) formation of smaller, more uniformly distributed macromolecular or nanoscopic domains. The uniform expansion (scenario 1) can be eliminated based on theoretical considerations; specifically, the aforementioned FEM calculations indicate that the additional surface area created over the deforming portion of the wire is only a fraction of a percent (cf. Figure S7) of the original area, and the average increase in the thiol-to-thiol distance would thus be no more than ∼1%. Because the thiols in the tightly packed SAMs are separated23,24 by ∼0.5 nm, this increase would translate into only a few picometers, much too small to open a “gap” into which a molecule of H2O2 could fit. To distinguish between the remaining scenarios (2) and (3), we considered a number of experimental techniques (e.g., ellipsometry, confocal microscopy, atomic force microscopy, etc.; see also the SI, section 6), ultimately choosing Kelvin probe force microscopy (KPFM), which is known30,31 to provide a high contrast of the surface potential between regions of unprotected metal and regions of metal covered with thiolate SAMs. For the ease of fabrication and availability of relevant

background studies,30,31 we chose an analogous system of 1naphthalenethiols on gold (Au) rather than on Pt. We prepared a deformable substrate by spin-coating poly(methyl methacrylate) (PMMA; 1 min at 2000 rpm; Sigma-Aldrich, average molecular weight ∼ 120000, catalog no. 182230) onto a 250μm-thick, market-grade polyethylene substrate and e-beamdepositing 100 nm of Au. When this planar surface was bent, we had only one radius of curvature to consider (as opposed to two radii for a thin wire), which simplified KPFM analysis. Finally, to delineate bare and SAM-covered regions, we microcontact-printed an array of thin (6 μm spaced by 8 μm) lines of the SAM. In the unbent state, the KPFM images of lines appeared sharp (Figure 4a). The contrast between the Au and Au/SAM regions was reflected by the distribution of the surface potentials featuring two distinct peaks: one at lower potentials corresponding to bare Au and one at higher potentials corresponding to Au/SAM (Figure 4b). When, however, the surface was bent, the signal over the SAM-covered region moved to lower potentials (Figure 4c) and the two peaks of the potential distribution merged into one peak at “intermediate” potential values (Figure 4d). This means that the SAM became more sparse and effectively exposed some underlying Au. We emphasize that no distinct cracks were observed [scenario (2)], substantiating instead the formation of uniformly distributed unprotected metal domains of dimensions below the ∼10 nm resolution of the KPFM system (Bruker Dimension Icon operating with a PFQNE-AL tip). Finally, when the surface was allowed to unbend (Figure 4e), the distribution of the potential values reverted to bimodal (Figure 4f), although it was not fully reversible because some thiols could have irreversibly migrated to the free-Au regions32 (note: in the case of the Pt wire, such net transport is irrelevant because the entire metal surface is covered with thiols). In summary, the above experiments corroborate “opening up” of nanoscopic domains within the SAM during mechanical deformation of the underlying metal support. At present, we do not know how these domains look at the atomic level, in terms of grain boundaries/defects; obtaining such an understanding would be an interesting, albeit possibly very challenging, goal 44267

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ACS Applied Materials & Interfaces

(11) Ueno, A.; Takahashi, K.; Osa, T. Photocontrol of Catalytic Activity of Capped Cyclodextrin. J. Chem. Soc., Chem. Commun. 1981, 3, 94−96. (12) Ueno, A.; Takahashi, K.; Osa, T. Photoregulation of Catalytic Activity of β-Cyclodextrin by an Azo Inhibitor. J. Chem. Soc., Chem. Commun. 1980, 17, 837−838. (13) Berryman, O. B.; Sather, A. C.; Lledó, A.; Rebek, J. Switchable Catalysis with a Light-Responsive Cavitand. Angew. Chem., Int. Ed. 2011, 50, 9400−9403. (14) Blanco, V.; Carlone, A.; Hänni, K. D.; Leigh, D. A.; Lewandowski, B. A Rotaxane-Based Switchable Organocatalyst. Angew. Chem., Int. Ed. 2012, 51, 5166−5169. (15) Imahori, T.; Yamaguchi, R.; Kurihara, S. Azobenzene-Tethered Bis(Trityl Alcohol) as a Photoswitchable Cooperative Acid Catalyst for Morita−Baylis−Hillman Reactions. Chem. - Eur. J. 2012, 18, 10802−10807. (16) Zirngast, M.; Pump, E.; Leitgeb, A.; Albering, J. H.; Slugovc, C. Pyridine as Trrigger for Chloride Isomerisation in Chelated Ruthenium Benzylidene Complexes: Implications for Olefin Metathesis. Chem. Commun. 2011, 47, 2261−2263. (17) De, S.; Pramanik, S.; Schmittel, M. A Monomer−Dimer Nanoswitch that Mimics the Working Principle of the SARS-CoV 3CLpro Enzyme Controls Copper-catalysed Cyclopropanation. Dalton Trans. 2014, 43, 10977−10982. (18) Blanco, V.; Leigh, D. A.; Marcos, V. Artificial Switchable Catalysts. Chem. Soc. Rev. 2015, 44, 5341−5370. (19) Wei, Y.; Han, S.; Kim, J.; Soh, S.; Grzybowski, B. A. Photoswitchable Catalysis Mediated by Dynamic Aggregation of Nanoparticles. J. Am. Chem. Soc. 2010, 132, 11018−11020. (20) Karthikeyan, S.; Potisek, S. L.; Piermattei, A.; Sijbesma, R. P. Highly Efficient Mechanochemical Scission of Silver-Carbene Coordination Polymers. J. Am. Chem. Soc. 2008, 130, 14968−14969. (21) Piermattei, A.; Karthikeyan, S.; Sijbesma, R. P. Activating Catalysts with Mechanical Force. Nat. Chem. 2009, 1, 133−137. (22) Jakobs, R. T. M.; Ma, S.; Sijbesma, R. P. Mechanocatalytic Polymerization and Cross-Linking in a Polymeric Matrix. ACS Macro Lett. 2013, 2, 613−616. (23) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1170. (24) Witt, D.; Klajn, R.; Barski, P.; Grzybowski, B. A. Applications, Properties and Synthesis of ω-Functionalized n-Alkanethiols and Disulfides - The Building Blocks of Self-Assembled Monolayers. Curr. Org. Chem. 2004, 8, 1763−1797. (25) Love, J. C.; Wolfe, D. B.; Haasch, R.; Chabinyc, M. L.; Paul, K. E.; Whitesides, G. M.; Nuzzo, R. G. Formation and Structure of SelfAssembled Monolayers of Alkanethiolates on Palladium. J. Am. Chem. Soc. 2003, 125, 2597−2609. (26) Vetter, T. A.; Colombo, D. P. Kinetics of Platinum-Catalyzed Decomposition of Hydrogen Peroxide. J. Chem. Educ. 2003, 80, 788− 789. (27) Panigrahi, S.; Basu, S.; Praharaj, S.; Pande, S.; Jana, S.; Pal, A.; Ghosh, S. K.; Pal, T. Synthesis and Size-Selective Catalysis by Supported Gold Nanoparticles: Study on Heterogeneous and Homogeneous Catalytic Process. J. Phys. Chem. C 2007, 111, 4596− 4605. (28) Kibler, L. A.; El-Aziz, A. M.; Hoyer, R.; Kolb, D. M. Tuning Reaction Rates by Lateral Strain in a Palladium Monolayer. Angew. Chem., Int. Ed. 2005, 44, 2080−2084. (29) We note that the activities of bare Pt wires did not change perceptibly upon different precleaning treatments (mechanic polishing or acid treatment in aqua regia; see the SI, section 1). No systematic differences in the amounts of oxygen produced were observed. (30) Campiglio, P.; Campione, M.; Sassella, A. Kelvin Probe Force Microscopy Characterization of Self-Assembled Monolayers on Metals Deposited with Dip-Pen Nanolithography. J. Phys. Chem. C 2009, 113, 8329−8335.

for future research. We believe the concept of mechanically controlled heterogeneous catalysis that we described here can be extended to other metal/SAM systems (or polymer-covered metals), perhaps in the context of sequential catalysis, whereby different catalysts would be activated/deactivated at various times. The major limitation of the current system is that the reversibility of catalysis appears to depend crucially on the quality of the SAM (cf. the data in Figure 2b), which, however, could be improved by decreasing the surface roughness of the metal support.33

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15253.

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Additional experimental details and FEM analyses (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bartosz A. Grzybowski: 0000-0001-6613-4261 Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge generous funding from the Institute for Basic Science Korea (Project Code IBS-R020-D1).

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REFERENCES

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DOI: 10.1021/acsami.7b15253 ACS Appl. Mater. Interfaces 2017, 9, 44264−44269