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Mechano-Catalysis: Cyclohexane Oxidation in a Silver Nanowire Break Junction Duncan den Boer,† Oleg I. Shklyarevskii,†,‡ Michiel J. J. Coenen,† Minko van der Maas,† Theo P. J. Peters,† Johannes A. A. W. Elemans,*,† and Sylvia Speller*,† † ‡
Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands B. Verkin Institute for Low Temperature Physics & Engineering, National Academy Of Science of Ukraine, 47 Lenin Av., 61103 Kharkov, Ukraine ABSTRACT: The dissociation of molecular oxygen at the clean surface of silver nanowires is studied in a mechanically controllable break junction (MCBJ) operating in organic liquids. From conductance histogram measurements, it can be concluded that the breaking of the nanowires exposes clean surfaces at which dissociation of dissolved oxygen molecules takes place, followed by chemisorption of the oxygen atoms at the nanowire surface. Subsequent characteristic changes in the appearance of individual conductance curves in the tunneling regime point to the mechano-catalytic oxidation of the solvent cyclohexane to cyclohexanol, and it is proposed that the oxygen atoms adsorbed at the nanowires are activated for this reaction, with the MCBJ acting as the mechano-catalyst.
’ INTRODUCTION Over the last years, charge transport properties of single molecules have been extensively studied in an attempt to develop active electronic components.1 3 The mechanically controllable break junction (MCBJ) technique is one of the most popular methods to measure molecular conductances.4 7 An attractive alternative approach is to use modified scanning tunneling microscopy (STM) setups, which mainly operate in organic solvents.8 10 Both techniques offer the possibility of statistically analyzing molecules based on the conductance histogram technique.11 Care, however, needs to be taken, since insufficient precautions against contamination of the electrode surface and its environment (e.g., a solvent) can lead to a large scattering of the conductance data (factors from 2 to 50), as has been reported by several groups and is apparent from a large disagreement between the experimental results and the theoretical calculations.12,13 Not surprisingly, such contaminations can lead to degradation of the junctions after a certain time or number of connection disconnection cycles.14,15 So far, nearly all MCBJ measurements have been carried out using gold electrodes, under the assumption that they are chemically rather inert. This inertness, however, is only true for the bulk form of gold; in the form of nanoparticles, clusters, and as a nanoporous material, the metal can be catalytically active in a variety of chemical reactions.16 22 Gold nanowires and in particular single atom chains can be even more chemically active.23 25 The increase of catalytic activity of metal particles upon their decrease in size is not only limited to gold. Silver, a metal very similar to gold in terms of atomic and electronic structure, has some peculiar properties. Its antibacterial behavior26 and its activity as a heterogeneous catalyst for the epoxidation of r 2011 American Chemical Society
ethene27 are well-known, and the interaction of oxygen with silver surfaces has recently drawn considerable attention.28 31 Using the histogram technique, Van Ruitenbeek et al. have studied the influence of oxygen on the conductance of silver nanowires and the length of the atomic chains at 4.2 and 40 K.32 It was suggested that dissociation of the oxygen molecules occurred at the silver nanowires, followed by incorporation of the oxygen atoms into the atomic chains. Here, we report on our first results on the behavior of a silver nanowire MCBJ in a cyclohexane solvent at room temperature, emphasizing the interaction of the nanowires with dissolved molecular oxygen. More specifically, we describe the unprecedented ability of these silver nanowires to act as a “mechano-catalyst”, by dissociating molecular oxygen present in the solution by the MCBJ and thereby activating it for the catalytic oxidation of alkane solvent molecules.
’ EXPERIMENTAL SECTION A standard MCBJ sample mounting, that is, a notched wire glued with hard epoxy to a flexible bending beam, was used, giving a moderate aspect ratio (ratio between the vertical motion perpendicular to the MCBJ axis and the distance z between the electrodes) of about 300 1000 and the possibility of using a standard piezodriver for repeated connection disconnection cycles. For this reason, the collection of statistical data is much faster than when a litographically made MCBJ is used, and this is especially important if the properties of the solvent or of the electrode surfaces change during the measurements as a result of Received: December 2, 2010 Revised: February 28, 2011 Published: March 31, 2011 8295
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Figure 1. (a) Typical conductance histogram for a silver nanowire in distilled and degassed cyclohexane; inset: conductance histogram for silver in UHV at 4.2 K. (b) Alternative conductance histogram (occurring in 30 40% of the samples) showing dominant peaks at higher conductance (see text). (c) The same conductance histogram as in panel a but on a logarithmic scale. (d) Typical conductance histogram (logarithmic scale) for a silver nanowire after replacing 10 20% of the degassed cyclohexane solvent with oxygen-saturated cyclohexane.
a chemical reaction. The samples were sonically cleaned and stored (for days or even weeks) in the solvent under investigation of high-performance liquid chromatography grade. Conductance measurements were performed in a quartz glass liquid cell containing 5 cm3 of solvent, which was hermetically closed with a Teflon lid. This setup allowed the possibility to work with volatile solvents for at least 48 72 h. The contents of the cell can be partly changed during the process of sample measurements. Further details of the experimental setup can be found elsewhere.33 Prior to use, all solvents were purified by distillation and degassed by three freeze pump thaw cycles when applicable.
’ RESULTS AND DISCUSSION First, the characteristics of the silver MCBJ were investigated at room temperature in cyclohexane that had been distilled and degassed by three freeze pump thaw cycles. Typical conductance histograms measured under these conditions are presented in Figure 1a,c. They differ significantly from histograms observed for the same MCBJ measured at 4.2 K in cryogenic vacuum (inset in Figure 1a), and the position and relative intensity of the peaks in the liquid experiment are strongly affected by the atomic shell effect in silver at room temperature.34,35 In a relatively large number of MCBJ experiments (∼30 40%), highly uncharacteristic conductance histograms were observed (Figure 1b) using the same experimental procedure as for histograms presented in Figure 1a. For these junctions, the single-atom peak as well as peaks corresponding to 2 5 atoms in the nanowire cross-section are very weak. This means that the jumplike transition from direct contact to tunneling occurs via atomic arrangements with a rather high conductance, involving the breaking of rather “thick” nanowires. Such effects were never observed under cryogenic
ultrahigh vacuum (UHV) conditions, in which the one-atom peak is dominant for all noble metals.36 Similar to our liquid experiments, a strong redistribution of peak intensities in the conductance histograms was also observed for silver nanowires in UHV at room temperature,37 which was attributed to the atomic structure of the nanowires along the chosen crystallographic direction. Although the crystallographic orientation of the electrodes at the breaking point of the polycrystalline wire is random and unknown, we believe that the observed effects can be related to some specific crystallographic directions and may be affected by the finite viscosity of the solvent. All of the following results were obtained exclusively on silver contacts with a highly intense, dominant single-atom peak, which indicates a very clean surface and a high probability of pulling (short) atomic chains in the process of electrode disconnection. The conductance histograms, in particular in the region of subquantum conductances, were found to change drastically at the moment that 10 20% of the cyclohexane in the cell was replaced by oxygen-saturated cyclohexane (Figure 1d). In comparison to the conductance histogram measured in degassed cyclohexane (Figure 1c), the relative amplitude of the single atom peak at ∼1 G0 decreased in intensity, and concomitantly, a new feature at 0.4 G0 appeared. The frequency of this new feature was found to vary from one experiment to another by almost an order of magnitude, and the number of the related individual conductance traces displaying steps in the range 0.3 0.5 G0 ranged from 5 to 50%. Those traces were automatically selected from the data set, including only conductance curves containing at least 10 50 data points between 0.3 and 0.5 G0, depending on individual contact properties. In the conductance histogram composed of the selected traces, it can be immediately seen that the peaks at ∼0.4 G0 and ∼1 G0 are anticorrelated, as the 8296
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Figure 2. Conductance histograms of selected traces of the set of histograms in Figure 1d. The disappearance of the single-atom peak at ∼1 G0 is accompanied by an increase in intensity of the peak around 1.3 G0, indicated by the arrow. Insets: schematic representation of an oxygen atom bridging a short chain of silver atoms before (right) and after (left) the break of the MCBJ.
intensity of the single-atom peak almost drops to background level (Figure 2). The peak around 1.3 G0 increases, which indicates that the conductance through the contact prior to the break exceeds one quantum unit. The observed conductances in the histogram upon the introduction of molecular oxygen can be explained as follows: O2 molecules can dissociate on the surface of the silver nanowire, and subsequently, the atomic oxygen can be chemisorbed.38 An oxygen atom can be chemisorbed in a bridging geometry, connecting two neighboring silver atoms (right structure in the inset in Figure 2).39 When oxygen bridges the single atom contact prior to the break, the total conductance through such an arrangement is the sum of the single atom conductance (∼1 G0) and the oxygen atom conductance and, hence, exceeds one quantum unit. After disconnection of the electrodes, the oxygen atom might remain attached to both of them, essentially bridging the gap (left structure in the inset in Figure 2). The peak at 0.4 G0 in the conductance histogram is probably related to such a situation and corresponding to the conduction through this oxygen atom. In previous research dealing with silver break junctions, additional conductances at 0.2 G0 were observed, which were attributed to oxygen atoms that were incorporated in a chain of silver atoms.28 31 The lower conductance value as compared to the one observed in our system is attributed to the fact that in the previously described atomic chains at least two oxygen atoms were present, as opposed to the proposed single oxygen atom in the present case. Another study revealed that molecular oxygen can chemisorb to silver clusters, involving a charge transfer interaction from the silver atom to the antibonding π* orbital of O2.31,40 We do not expect a significant influence of this type of interaction on the conductance of nanowires, since the electrodes cannot be bridged by an oxygen molecule (as in that case the oxygen molecule would dissociate41,42). However, O2 chemisorption could contribute to the contamination of the surface and deterioration of the conductance histograms. Dissociation of an oxygen molecule may not only result in the chemisorption of atomic oxygen on the surface of the silver electrodes, but it can lead to a subsequent catalytic oxidation of
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Figure 3. (a) Typical conductance traces for a silver MCBJ in (traces 1 and 2) degassed cyclohexane and (trace 3) cyclohexane with dissolved oxygen. (b) Conductance traces for a silver MCBJ in the polar liquids ethanol (trace 1), cyclohexanol (trace 2), benzyl alcohol (trace 3), and cyclohexanone (trace 4).
the cyclohexane solvent to cyclohexanol and/or cyclohexanone.43,44 These products are important precursors for the production of apidic acid, and the oxidation of cyclohexane has been studied using a wide variety of catalysts.45 Both cyclohexanol and cyclohexanone are relatively polar molecules, so if they are catalytically generated during the MCBJ process, significant changes can be expected in the contact conductance in the tunneling regime. During the break junction experiments, indeed in approximately 10 15% of the cases, conductance traces in the connection disconnection cycle were observed, which are compatible with the presence of one of the above-mentioned polar compounds. Conductance traces 1 and 2 in Figure 3a are typically observed when the break junction measurements are carried out in oxygen-free cyclohexane. In that case, the conductance drops to a value below the detection level (10 4 G0), either by a sudden jump from the direct mechanical contact or by an exponential decrease after an initial jump to 10 1 10 3 G0. When oxygen is present in the liquid, dramatically different conductance traces were observed (cf. trace 3 in Figure 3a). After an initial drop in conductance to ∼0.01 G0, the conductance does not decrease more than 1 order of magnitude during a 1.5 2.0 nm further displacement of the electrodes, with a gentle downhill slope in the trace. We found that such a curve shape is characteristic for break junction measurements in polar liquids; this is illustrated in Figure 3b, which shows the measured conductance curves in the pure polar solvents ethanol, cyclohexanol, benzyl alcohol, and cyclohexanone. It is highly remarkable that the conductance curves in pure cyclohexanol (trace 2 in Figure 3b) closely resemble the curves observed for the experiments in oxygencontaining cyclohexane (trace 3 in Figure 3a). We therefore propose that during the break junction experiment in the latter solvent the silver nanowire MCBJ acts as a novel type of “mechano-catalyst” (for some other types of mechano-catalysis see refs 46 49), which 8297
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The Journal of Physical Chemistry C oxidizes cyclohexane molecules to give cyclohexanol as a product. The mechanic connection disconnection cycles of the break junction lead to the continuous formation of nanowires, which upon breaking expose clean metallic surfaces, which are capable of catalyzing the oxidation reaction at certain atomic configurations. One of the other possible products of the catalytic oxidation of cyclohexane, cyclohexanone, gives rise to a conductance which is 2 orders of magnitude lower than we observe (trace 4 in Figure 3b), and for that reason, its formation during the MCBJ experiments cannot explain the observed effects. Cyclohexane could be produced also but would not be visible because of the presence of cyclohexanol. Although only very small amounts of oxidized molecules will be generated, it is possible that these will form a small, polar cluster, which is trapped in the highly confined space and electric field between a particular configuration of the two electrodes of the break junction. Under these circumstances, such polar clusters can contribute significantly to the conductance in the tunneling range. Such a situation would be highly inhomogeneous, and when the separation between the electrodes increases beyond the cluster size, it can be expected that the conductance behavior reverts to resemble the exponential again. The onset of such behavior is clearly visible in trace 3 in Figure 3a. An interruption of the break junction experiment for 5 10 min results in the recovery of conductance behavior that is typically observed for nonpolar liquids, which is proposed to be caused by either diffusion of the polar oxidation products away from the junction or their adsorption to the surface of one of the silver electrodes. The effect of product formation became visible in the conductance histogram after 1000 3000 connection disconnection cycles of the MCBJ. If (i) it is assumed that all of the produced cyclohexanol molecules stay in the vicinity of the single atom contact, (ii) it is estimated that for its detection the size of the polar liquid volume requires the presence of 30 100 of these molecules, and (iii) the operating frequency of the break junction (10 Hz) is considered, an average reaction rate between 1 molecule/s and 1 molecule/10 s can be estimated. Continuous chemisorption of oxygen atoms on the silver surface can in principle lead to the formation of a full oxygen layer. In the absence of O2, deterioration of the quality of the conductance histograms usually occurs at a slow rate; in distilled and outgassed solvent, no visible changes occur in the data in 24 h or after 8 h of continuous measurements (repeatable indenting withdrawal cycles at a 10 Hz rate). This indicates that the electrodes remain relatively clean. In contrast, in the presence of molecular oxygen, the mechanical stability of the junctions drops considerably during the course of the measurement. In many occasions, after 10000 20000 cycles, the electrodes spontaneously snap together or move apart beyond the range of the piezodriver. Both phenomena are related to changes in electrode geometry at a scale that exceeds 10 nm. The changes in mechanical properties of the silver nanowires may be additional indirect evidence for electrode oxidation, a reaction of the electrodes with the product, or side products of the alkane oxidation.
’ CONCLUSIONS In conclusion, molecular oxygen dissolved in the nonpolar cyclohexane can dissociate at the clean surface of a silver nanowire that is produced in the process of breaking electrodes in a MCBJ. Chemisorption of atomic oxygen subsequently results in changes of the conductance histogram of silver, including the
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appearance of a conductance feature at 0.4 G0, which is related to the conductance through an oxygen atom. Subsequent characteristic changes in the appearance of individual conductance curves in the tunneling regime point to the mechano-catalytic oxidation of the cyclohexane solvent to cyclohexanol.
’ ACKNOWLEDGMENT We are grateful to J. G. H. Hermsen and J. W. Gerritsen for invaluable technical assistance, and J. M. van Ruitenbeek, A. E. Rowan, and R. J. M. Nolte for stimulating discussions. Part of this work was supported by the Nanotechnology network in The Netherlands (NanoNed), by the Ministry of Economic Affairs, and the Stichting voor Fundamenteel Onderzoek der Materie (FOM), which is financially supported by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). J.A.A.W.E. wishes to thank NWO for a Vidi Grant (700.58.423) and the European Research Council for an ERC Starting Grant (NANOCAT 259064), and O.I.S. thanks FOM and NWO for a visitor's grant. ’ REFERENCES (1) Akkerman, H. B.; De Boer, B. J. Phys.: Condens. Matter 2008, 20, 013001. (2) Selzer, Y.; Allara, D. L. Annu. Rev. Phys. Chem. 2006, 57, 593. (3) Chen, F.; Hihath, J.; Huang, Z.; Li, X.; Tao, N. J. Annu. Rev. Phys. Chem. 2007, 58, 535. (4) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (5) Smit, R. H. M.; Noat, Y.; Untiedt, C.; Lang, N. D.; Van Hemert, M. C.; Van Ruitenbeek, J. M. Nature 2002, 419, 906. (6) Csonka, S.; Halbritter, A.; Mihaly, G.; Shklyarevskii, O. I.; Speller, S.; Van Kempen, H. Phys. Rev. Lett. 2004, 93, 016802. (7) Van Ruitenbeek, J. M.; Scheer, E.; Weber, H. In Introducing Molecular Electronics; Cuniberti, G., Fagas, G., Richter, K., Eds.; Springer Lecture Notes in Physics; Springer: Heidelberg, 2005; Chapter Contacting individual molecules using mechanically controllable break junction, p 253. (8) Xu, B.; Tao, N. J. Science 2003, 301, 1221. (9) Haiss, W.; Nichols, R. J.; Van Zalinge, H.; Higgins, S. J.; Bethell, D.; Schiffrin, D. J. Phys. Chem. Chem. Phys. 2004, 6, 4330. (10) Venkataraman, L.; Klare, J. E.; Tam, I. W.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nano Lett. 2006, 6, 458. (11) Agrait, N.; Yeyati, A. L.; Van Ruitenbeek, J. M. Phys. Rep. 2003, 377, 81. (12) Lindsay, S. M.; Ratner, M. A. Adv. Mater. 2007, 19, 23. (13) Ulrich, J.; Esrail, D.; Pontius, W.; Venkataraman, L.; Millar, D.; Doerrer, L. H. J. Phys. Chem. B 2006, 110, 2462. (14) Millar, D.; Venkataraman, L.; Doerrer, L. H. J. Phys. Chem. C 2007, 111, 17635. (15) Long, D. P.; Lazorcik, J. L.; Mantooth, B. A.; Moore, M. H.; Ratner, M. A.; Troisi, A.; Yao, Y.; Ciszek, J. W.; Tour, J. M.; Shashidhar, R. Nat. Mater. 2006, 5, 901. (16) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (17) Hughes, M. D.; Xu, Y. J.; Jenkins, P.; McMorn, P.; Landon, P.; Enache, D. I.; Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; Stitt, E. H.; Johnston, P.; Griffin, K.; Kiely, C. J. Nature 2005, 437, 1132. (18) Corma, A.; Garcia, H. Chem. Soc. Rev. 2008, 37, 2096. (19) Chen, M.; Goodman, D. W. Chem. Soc. Rev. 2008, 37, 1860. (20) Novo, C.; Funston, A. M.; Mulvaney, P. Nat. Nanotechnol. 2008, 3, 598. (21) Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; B€aumer, M. Science 2010, 327, 319. (22) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896. 8298
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